m_abs_lookup.cc

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/* Copyright (C) 2002-2008 Stefan Buehler <sbuehler@ltu.se>
  
   This program is free software; you can redistribute it and/or
   modify it under the terms of the GNU General Public License as
   published by the Free Software Foundation; either version 2 of the
   License, or (at your option) any later version.
  
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.
  
   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
   USA. */
 
#include <algorithm> 
#include <map>
#include <limits>
 
#include "auto_md.h"
#include "arts.h"
#include "messages.h"
#include "gas_abs_lookup.h"
#include "agenda_class.h"
#include "check_input.h"
#include "matpackV.h"
#include "physics_funcs.h"
#include "math_funcs.h"
#include "make_vector.h"
#include "arts_omp.h"
#include "interpolation_poly.h"
#include "rng.h"
 
extern const Index GFIELD4_FIELD_NAMES;
extern const Index GFIELD4_P_GRID;
 
 
/* Workspace method: Doxygen documentation will be auto-generated */
void abs_lookupInit(GasAbsLookup& /* x */, const Verbosity& verbosity)
{
  ArtsOut2 out2(verbosity);
  // Nothing to do here.
  // That means, we rely on the default constructor.
 
  out2 << "  Created an empty gas absorption lookup table.\n";
}
 
 
/* Workspace method: Doxygen documentation will be auto-generated */
void abs_lookupCreate(// WS Output:
                      GasAbsLookup& abs_lookup,
                      Index& abs_lookup_is_adapted,
                      // WS Input:
                      const ArrayOfArrayOfSpeciesTag& abs_species,
                      const ArrayOfArrayOfLineRecord& abs_lines_per_species,
                      const ArrayOfLineshapeSpec&     abs_lineshape,
                      const ArrayOfArrayOfSpeciesTag& abs_nls,
                      const Vector&                   f_grid,
                      const Vector&                   abs_p,
                      const Matrix&                   abs_vmrs,
                      const Vector&                   abs_t,
                      const Vector&                   abs_t_pert,
                      const Vector&                   abs_nls_pert, 
                      const Vector&                   abs_n2,            
                      const ArrayOfString&            abs_cont_names,    
                      const ArrayOfString&            abs_cont_models,   
                      const ArrayOfVector&            abs_cont_parameters,
                      const Verbosity&                verbosity)
{
  CREATE_OUT2
  CREATE_OUT3
  
  // We need this to restore the original setting of omp_nested at the
  // end.
  int omp_nested_original = arts_omp_get_nested();
 
  // We will be calling an absorption agenda one species at a
  // time. This is better than doing all simultaneously, because is
  // saves memory and allows for consistent treatment of nonlinear
  // species. But it means we need local copies of species, line list,
  // and line shapes for agenda communication.
  
  // 1. Output of absorption calculations:
 
  // Absorption coefficients:
  Matrix these_abs_coef;
 
  // Absorption cross sections per tag group. 
  ArrayOfMatrix abs_xsec_per_species;
 
 
  // 2. Determine various important sizes:
  const Index n_species = abs_species.nelem();   // Number of abs species
  const Index n_nls = abs_nls.nelem();           // Number of nonlinear species
  const Index n_f_grid = f_grid.nelem();         // Number of frequency grid points
  const Index n_p_grid = abs_p.nelem();          // Number of presure grid points
  const Index n_t_pert = abs_t_pert.nelem();     // Number of temp. perturbations
  const Index n_nls_pert = abs_nls_pert.nelem(); // Number of VMR pert. for NLS
 
  // 3. Input to absorption calculations:
 
  // Absorption vmrs and temperature:
  Matrix this_vmr(1,n_p_grid);
  Vector abs_h2o(n_p_grid);
  Vector this_t;                // Has same dimension, but is
                                // initialized by assignment later.
 
  // Species list, lines, and line shapes, all with only 1 element:
  ArrayOfArrayOfSpeciesTag this_species(1);
  ArrayOfArrayOfLineRecord these_lines(1);
  ArrayOfLineshapeSpec this_lineshape(1);
 
  // Local copy of nls_pert and t_pert:
  Vector these_nls_pert;        // Is resized every time when used
  Vector these_t_pert;          // Is resized later on
 
  // 4. Checks of input parameter correctness:
 
  const Index h2o_index 
    = find_first_species_tg( abs_species,
                             species_index_from_species_name("H2O") );
 
  if ( h2o_index < 0 )
    {
      // If there are nonlinear species, then at least one species must be
      // H2O. We will use that to set h2o_abs, and to perturb in the case
      // of nonlinear species.
      if (n_nls>0)
        {
          ostringstream os;
          os << "If you have nonlinear species, at least one\n"
             << "species must be a H2O species.";
          throw runtime_error( os.str() );
        }
      else
        {
          out2 << "  You have no H2O species. Absorption continua will not work.\n"
               << "  You should get a runtime error if you try them anyway.\n";
        }
    }
 
  // abs_species, f_grid, and p_grid should not be empty:
  if ( 0==n_species ||
       0==n_f_grid ||
       0==n_p_grid )
    {
      ostringstream os;
      os << "One of the required input variables is empty:\n"
         << "abs_species.nelem() = " << n_species << ",\n"
         << "f_grid.nelem() = " << n_f_grid << ",\n"
         << "abs_p.nelem() = " << n_p_grid << ".";
      throw runtime_error( os.str() );
    }
 
  // Set up the index array abs_nls from the tag array
  // abs_nls. Give an error message if these
  // tags are not included in abs_species. 
  ArrayOfIndex abs_nls_idx;  
  for (Index i=0; i<n_nls; ++i)
    {
      Index s;
      for (s=0; s<n_species; ++s)
        {
          if (abs_nls[i]==abs_species[s])
            {
              abs_nls_idx.push_back(s);
              break;
            }
        }
      if (s==n_species)
        {
          ostringstream os;
          os << "Did not find *abs_nls* tag group \""
             << get_tag_group_name(abs_nls[i])
             << "\" in *abs_species*.";
          throw runtime_error( os.str() );
        }
    }
 
  // Furthermore abs_nls_idx must not contain duplicate values:
  if ( !is_unique(abs_nls_idx) )
    {
      ostringstream os;
      os << "The variable *abs_nls* must not have duplicate species.\n"
         << "Value of *abs_nls*: " << abs_nls_idx;
      throw runtime_error( os.str() );
    }
 
  // abs_lines_per_species must match abs_species:
  if (abs_lines_per_species.nelem() != abs_species.nelem())
    {
      ostringstream os;
      os << "Dimensions of *abs_species* and *abs_lines_per_species* must match. "
         << "*abs_species*: " << abs_species.nelem() << " elements. "
         << "*abs_lines_per_species*: " << abs_lines_per_species.nelem() << " elements.";
      throw runtime_error( os.str() );      
    }
 
  // VMR matrix must match species list and pressure levels:
  chk_size( "abs_vmrs",
            abs_vmrs,
            n_species,
            n_p_grid );
 
  // Temperature vector must match number of pressure levels:
  chk_size( "abs_t",
            abs_t,
            n_p_grid ); 
 
  // abs_nls_pert should only be not-empty if we have nonlinear species:
  if ( ( (0==n_nls) && (0!=n_nls_pert) ) ||
       ( (0!=n_nls) && (0==n_nls_pert) ) )
    {
      ostringstream os;
      os << "You have to set both abs_nls and abs_nls_pert, or none.";
      throw runtime_error( os.str() );
    }
 
 
  // 4.a Set up a logical array for the nonlinear species.
  ArrayOfIndex non_linear(n_species,0);
  for ( Index s=0; s<n_nls; ++s )
    {
      non_linear[abs_nls_idx[s]] = 1;
    }
 
 
  // 5. Set general lookup table properties:
  abs_lookup.species = abs_species;    // Species list
  abs_lookup.nonlinear_species = abs_nls_idx;  // Nonlinear species   (e.g., H2O, O2)
  abs_lookup.f_grid = f_grid;           // Frequency grid
  abs_lookup.p_grid = abs_p;          // Pressure grid
  abs_lookup.vmrs_ref = abs_vmrs;
  abs_lookup.t_ref = abs_t;
  abs_lookup.t_pert = abs_t_pert;
  abs_lookup.nls_pert = abs_nls_pert;
 
  // 5.a. Set log_p_grid:
  abs_lookup.log_p_grid.resize(n_p_grid);
  transform( abs_lookup.log_p_grid, log, abs_lookup.p_grid );
 
 
  // 6. Create abs_lookup.xsec with the right dimensions:
  {
    Index a,b,c,d;
 
    if ( 0 == n_t_pert ) a = 1;
    else a = n_t_pert;
 
    b = n_species + n_nls * ( n_nls_pert - 1 );
 
    c = n_f_grid;
 
    d = n_p_grid;
 
    abs_lookup.xsec.resize( a, b, c, d );
  }
 
  // 6.a. Set up these_t_pert. This is done so that we can use the
  // same loop over the perturbations, independent of
  // whether we have temperature perturbations or not.
  if ( 0!=n_t_pert)
    {
      out2 << "  With temperature perturbations.\n";
      these_t_pert.resize(n_t_pert);
      these_t_pert = abs_t_pert;
    }
  else
    {
      out2 << "  No temperature perturbations.\n";
      these_t_pert.resize(1);
      these_t_pert = 0;
 
      // In this case, we also want to enable nested parallel
      // execution. Normally, it is the loop over the temperature
      // perturbations that is parallel, but in this case that one is
      // trivial. 
      arts_omp_set_nested(1);
    }
 
  const Index these_t_pert_nelem = these_t_pert.nelem();
  
 
  // 7. Now we have to fill abs_lookup.xsec with the right values!
 
  // Loop species:
  for ( Index i=0,spec=0; i<n_species; ++i )
    {
      // spec is the index for the second dimension of abs_lookup.xsec.
      
      // Prepare absorption agenda input for this species:
      out2 << "  Doing species " << i+1 << " of " << n_species << ": "
           << abs_species[i] << ".\n";
 
      // Get a dummy list of tag groups with only the current element:
      this_species[0].resize(abs_species[i].nelem());
      this_species[0] = abs_species[i];
 
      // List of lines:
      these_lines[0].resize(abs_lines_per_species[i].nelem());
      these_lines[0] = abs_lines_per_species[i];
      
      // List of lineshapes. This requires special treatment: If there
      // is only 1 lineshape given, the same lineshape should be used
      // for all species.
      if (1==abs_lineshape.nelem())
        this_lineshape[0] = abs_lineshape[0];
      else
        this_lineshape[0] = abs_lineshape[i];
 
      // Set up these_nls_pert. This is done so that we can use the
      // same loop over the perturbations, independent of
      // whether we have nonlinear species or not.
      if ( non_linear[i] )
        {
          out2 << "  This is a species with H2O VMR perturbations.\n";
          these_nls_pert.resize(n_nls_pert);
          these_nls_pert = abs_nls_pert;
        }
      else
        {
          these_nls_pert.resize(1);
          these_nls_pert = 1;
        }
      
      // Loop these_nls_pert:
      for ( Index s=0; s<these_nls_pert.nelem(); ++s,++spec )
        {
          // Remember, spec is the index for the second dimension of
          // abs_lookup.xsec
          
          if ( non_linear[i] )
            {
              out2 << "  Doing H2O VMR variant " << s+1 << " of " << n_nls_pert << ": "
                   << abs_nls_pert[s] << ".\n";
            }
 
          // VMR for this species:
          this_vmr(0,joker) = abs_vmrs(i,joker);  
          if ( i==h2o_index )
            {
              //              out3 << "  Species is main H2O species.\n";
              this_vmr(0,joker) *= these_nls_pert[s]; // Add perturbation
            }
 
          // For abs_h2o, we can always add the perturbations (it will
          // not make a difference if the species itself is also H2O).
          // Attention, we need to treat here also the case that there
          // is no H2O species. We will then set abs_h2o to
          // -1. Absorption routines that do not really need abs_h2o
          // will still run.
          if ( h2o_index == -1 )
            {
              // The case without H2O species.
              abs_h2o.resize(1);
              abs_h2o = -1;
            }
          else
            {
              // The normal case.
              abs_h2o = abs_vmrs(h2o_index, joker);   
              abs_h2o *= these_nls_pert[s]; // Add perturbation
            }
 
          // Loop temperature perturbations
          // ------------------------------
 
          // We use a parallel for loop for this. 
 
          // There is something strange here: abs_lookup seems to be
          // "shared" by default, although I have set default(none). I
          // suspect that the reason for this behavior is that
          // abs_lookup is a return by reference parameter of this
          // function. Anyway, shared is the correct setting for
          // abs_lookup, so there is no problem.
 
/*#pragma omp parallel for                                       \
  if(!arts_omp_in_parallel()                                   \
     && these_t_pert_nelem>=arts_omp_get_max_threads())        \
  default(none)                                                \
  shared(these_t_pert, out3, this_species,                     \
         this_lineshape, these_lines, this_vmr, abs_h2o,       \
         joker, spec, abs_lookup, abs_p, f_grid,               \
         abs_cont_models, abs_cont_parameters, abs_cont_names, \
         abs_n2)                                               \
  private(this_t, abs_xsec_per_species) */
#pragma omp parallel for                                       \
  if(!arts_omp_in_parallel()                                   \
     && these_t_pert_nelem>=arts_omp_get_max_threads())        \
  private(this_t, abs_xsec_per_species)
          for ( Index j=0; j<these_t_pert_nelem; ++j )
            {
              // The try block here is necessary to correctly handle
              // exceptions inside the parallel region. 
              try
                {
                  if ( 0!=n_t_pert )
                    {
                      // We first prepare the output in a string here,
                      // so that we can write it to out3 with a single
                      // operation. This avoids messy output from
                      // multiple threads.
                      ostringstream os;
 
                      os << "  Doing temperature variant " << j+1
                         << " of " << n_t_pert << ": "
                         << these_t_pert[j] << ".\n";
 
                      out3 << os.str();
                    }
              
                  // Create perturbed temperature profile:
                  this_t = abs_lookup.t_ref;
                  this_t += these_t_pert[j];
      
                  // The sequence of function calls here is inspired from
                  // abs_coefCalcSaveMemory. 
 
                  abs_xsec_per_speciesInit( abs_xsec_per_species, this_species,
                                            f_grid, abs_p, verbosity );
 
                  abs_xsec_per_speciesAddLines( abs_xsec_per_species,
                                                this_species,
                                                f_grid,
                                                abs_p,
                                                this_t,
                                                abs_h2o,
                                                this_vmr,
                                                these_lines,
                                                this_lineshape,
                                                verbosity);
 
                  abs_xsec_per_speciesAddConts( abs_xsec_per_species,
                                                this_species,
                                                f_grid,
                                                abs_p,
                                                this_t,
                                                abs_n2,
                                                abs_h2o,
                                                this_vmr,
                                                abs_cont_names,
                                                abs_cont_parameters,
                                                abs_cont_models,
                                                verbosity);
 
                  // Store in the right place:
                  // Loop through all altitudes
                  for ( Index p=0; p<n_p_grid; ++p )
                    {
                      abs_lookup.xsec( j, spec, Range(joker), p )
                        = abs_xsec_per_species[0](Range(joker),p);
 
                      // There used to be a division by the number density
                      // n here. This is no longer necessary, since
                      // abs_xsec_per_species now contains true absorption
                      // cross sections.
 
                      // IMPORTANT: There was a bug in my old Matlab
                      // function "create_lookup.m" to generate the lookup
                      // table. (The number density was always the
                      // reference one, and did not change for different
                      // temperatures.) Patricks Atmlab function
                      // "arts_abstable_from_arts1.m" did *not* have this bug.
 
                      // Calculate the number density for the given pressure and
                      // temperature: 
                      // n = n0*T0/p0 * p/T or n = p/kB/t, ideal gas law
                      //                  const Numeric n = number_density( abs_lookup.p_grid[p],
                      //                                                    this_t[p]   );
                      //                  abs_lookup.xsec( j, spec, Range(joker), p ) /= n;
                    }
                } // end of try block
              catch (runtime_error e)
                {
                  CREATE_OUT0
                  exit_or_rethrow(e.what(), out0);
                }
            } // end of parallel for loop
        }
    }
 
  // Set the abs_lookup_is_adapted flag. After all, the table fits the
  // current frequency grid and species selection.
  abs_lookup_is_adapted = 1;
 
  // Reset the omp_nested state to original value:
  arts_omp_set_nested(omp_nested_original);
}
 
 
 
void find_nonlinear_continua(ArrayOfIndex& cont,
                             const ArrayOfArrayOfSpeciesTag& abs_species,
                             const Verbosity& verbosity)
{
  CREATE_OUT3
  
  cont.resize(0);
  
  // This is quite complicated, unfortunately. The approach here
  // is copied from abs_xsec_per_speciesAddConts. For explanation,
  // see there.
 
  // Loop tag groups:
  for ( Index i=0; i<abs_species.nelem(); ++i )
    {
      extern const Array<SpeciesRecord> species_data; 
 
      // Loop tags in tag group
      for ( Index s=0; s<abs_species[i].nelem(); ++s )
        {
 
          // Check if this is a specific isotope, or "all". ("all" is
          // not a continuum tag.)
          if ( abs_species[i][s].Isotope() <
               species_data[abs_species[i][s].Species()].Isotope().nelem() )
            {
              // Continuum tags have isotope ratio -1
              if ( 0 >
                   species_data[abs_species[i][s].Species()].Isotope()[abs_species[i][s].Isotope()].Abundance() )
                {
                  const String thisname = abs_species[i][s].Name();
                  // Ok, now we know this is a continuum tag.
                  out3 << "  Continuum tag: " << thisname;
 
                  // Check whether we want nonlinear treatment for
                  // this or not. We have three classes of continua:
                  // 1. Those that we know do not require it
                  // 2. Those that we know require h2o_abs
                  // 3. Those for which we do not know
 
                  // The list here is not at all perfect, just a first
                  // guess. If you need particular models, you better
                  // check that the right thing is done with your model.
 
                  // 1. Continua known to not use h2o_abs
                  // We take also H2O itself here, since this is
                  // handled separately
                  if ( species_index_from_species_name("H2O")==abs_species[i][s].Species() ||
                       "N2-"==thisname.substr(0,3) ||
                       "CO2-"==thisname.substr(0,4) ||
                       "O2-CIA"==thisname.substr(0,6) ||
                       "O2-v0v"==thisname.substr(0,6) ||
                       "O2-v1v"==thisname.substr(0,6) )
                    {
                      out3 << " --> not added.\n";
                      break;
                    }
 
                  // 2. Continua known to use h2o_abs
                  if ( "O2-"==thisname.substr(0,3) )                       
                    {
                      cont.push_back(i);
                      out3 << " --> added to abs_nls.\n";
                      break;                      
                    }
 
                  // If we get here, then the tag was neither in the
                  // posivitive nor in the negative list. We through a
                  // runtime error.
                  out3 << " --> unknown.\n";
                  throw runtime_error("I don't know whether this tag uses h2o_abs or not.\n"
                                      "Cannot set abs_nls automatically.");
                }
            }              
        }
    }
}
 
 
 
void choose_abs_nls(ArrayOfArrayOfSpeciesTag& abs_nls,
                    const ArrayOfArrayOfSpeciesTag& abs_species,
                    const Verbosity& verbosity)
{
  CREATE_OUT2
  
  abs_nls.resize(0);
 
  // Add all H2O species as non-linear:
  Index next_h2o = 0;
  while (-1 != (next_h2o =
                find_next_species_tg(abs_species,
                                     species_index_from_species_name("H2O"),
                                     next_h2o)))
    {
      abs_nls.push_back(abs_species[next_h2o]);
      ++next_h2o;
    }
 
  // Certain continuum models also depend on abs_h2o. There is a
  // helper function that contains a list of these.
  ArrayOfIndex cont;
  find_nonlinear_continua(cont, abs_species, verbosity);
 
  // Add these to abs_nls:
  for (Index i=0; i<cont.nelem(); ++i)
    {
      abs_nls.push_back(abs_species[cont[i]]);
    }
 
  out2 << "  Species marked for nonlinear treatment:\n";
  for (Index i=0; i<abs_nls.nelem(); ++i)
    {
      out2 << "  ";
      for (Index j=0; j<abs_nls[i].nelem(); ++j)
        {
          if (j!=0) out2 << ", ";
          out2 << abs_nls[i][j].Name();
        }
      out2 << "\n";
    }
}
 
 
 
void choose_abs_t_pert(Vector&         abs_t_pert,
                       ConstVectorView abs_t,
                       ConstVectorView tmin,
                       ConstVectorView tmax,
                       const Numeric&  step,
                       const Index&    p_interp_order,
                       const Index&    t_interp_order,
                       const Verbosity& verbosity)
{
  CREATE_OUT2
  CREATE_OUT3
  
  // The code to find out the range for perturbation is a bit
  // complicated. The problem is that, since we use higher order
  // interpolation for p, we may require temperatures well outside the
  // local min/max range at the reference level. We solve this by
  // really looking at the min/max range for those levels required by
  // the p_interp_order.
  
  Numeric mindev = 1e9;
  Numeric maxdev = -1e9;
 
  Vector the_grid(0,abs_t.nelem(),1);
  for (Index i=0; i<the_grid.nelem(); ++i)
    {
      GridPosPoly gp;
      gridpos_poly(gp, the_grid, (Numeric)i, p_interp_order);
 
      for (Index j=0; j<gp.idx.nelem(); ++j)
        {
          // Our pressure grid for the lookup table may be coarser than
          // the original one for the batch cases. This may lead to max/min
          // values in the original data that exceed those we assumed
          // above. We add some extra margin here to account for
          // that. (The margin is +-10K)
 
          Numeric delta_min = tmin[i] - abs_t[gp.idx[j]] - 10;
          Numeric delta_max = tmax[i] - abs_t[gp.idx[j]] + 10;
 
          if ( delta_min < mindev ) mindev = delta_min;
          if ( delta_max > maxdev ) maxdev = delta_max;
        }
    }
 
  out3 << "  abs_t_pert: mindev/maxdev : " << mindev << " / " << maxdev << "\n";
 
  // We divide the interval between mindev and maxdev, so that the
  // steps are of size *step* or smaller. But we also need at least
  // *t_interp_order*+1 points.
  Index   div = t_interp_order;
  Numeric effective_step;
  do
    {
      effective_step = (maxdev-mindev)/(Numeric)div;
      ++div;
    }
  while (effective_step > step);
 
  abs_t_pert = Vector(mindev, div, effective_step);
 
  out2 << "  abs_t_pert: " << abs_t_pert[0] << " K to " << abs_t_pert[abs_t_pert.nelem()-1]
       << " K in steps of " << effective_step
       << " K (" << abs_t_pert.nelem() << " grid points)\n";
}
 
 
 
void choose_abs_nls_pert(Vector&         abs_nls_pert,
                         ConstVectorView refprof,
                         ConstVectorView minprof,
                         ConstVectorView maxprof,
                         const Numeric&  step,
                         const Index&    p_interp_order,
                         const Index&    nls_interp_order,
                         const Verbosity& verbosity)
{
  CREATE_OUT2
  CREATE_OUT3
  
  // The code to find out the range for perturbation is a bit
  // complicated. The problem is that, since we use higher order
  // interpolation for p, we may require humidities well outside the
  // local min/max range at the reference level. We solve this by
  // really looking at the min/max range for those levels required by
  // the p_interp_order.
 
  Numeric mindev = 0;
  Numeric maxdev = -1e9;
 
  // mindev is set to zero from the start, since we always want to
  // include 0.
 
  Vector the_grid(0,refprof.nelem(),1);
  for (Index i=0; i<the_grid.nelem(); ++i)
    {
//       cout << "!!!!!! i = " << i << "\n";
//       cout << " min/ref/max = " << minprof[i] << " / "
//            << refprof[i] << " / "
//            << maxprof[i] << "\n";
  
      GridPosPoly gp;
      gridpos_poly(gp, the_grid, (Numeric)i, p_interp_order);
 
      for (Index j=0; j<gp.idx.nelem(); ++j)
        {
//           cout << "!!!!!! j = " << j << "\n";
//           cout << "  ref[j] = " << refprof[gp.idx[j]] << "   ";
//           cout << "  minfrac[j] = " << minprof[i] / refprof[gp.idx[j]] << "   ";
//           cout << "  maxfrac[j] = " << maxprof[i] / refprof[gp.idx[j]] << "  \n";
 
          // Our pressure grid for the lookup table may be coarser than
          // the original one for the batch cases. This may lead to max/min
          // values in the original data that exceed those we assumed
          // above. We add some extra margin to the max value here to account for
          // that. (The margin is a factor of 2.)
 
          Numeric delta_min = minprof[i] / refprof[gp.idx[j]];
          Numeric delta_max = 2*maxprof[i] / refprof[gp.idx[j]];
 
          if ( delta_min < mindev ) mindev = delta_min;
          if ( delta_max > maxdev ) maxdev = delta_max;
        }
    }
 
  out3 << "  abs_nls_pert: mindev/maxdev : " << mindev << " / " << maxdev << "\n";
      
  bool allownegative = false;
  if (mindev<0) 
    {
      out2 << "  Warning: I am getting a negative fractional distance to the H2O\n"
           << "  reference profile. Some of your H2O fields may contain negative values.\n"
           << "  Will allow negative values also for abs_nls_pert.\n";
      allownegative = true;
    }
 
  if (!allownegative)
    {
      mindev = 0;
      out3 << "  Adjusted mindev : " << mindev << "\n";
    }
 
  // We divide the interval between mindev and maxdev, so that the
  // steps are of size *step* or smaller. But we also need at least
  // *nls_interp_order*+1 points.
  Index   div = nls_interp_order;
  Numeric effective_step;
  do
    {
      effective_step = (maxdev-mindev)/(Numeric)div;
      ++div;
    }
  while (effective_step > step);
 
  abs_nls_pert = Vector(mindev, div, effective_step);
 
  // If there are negative values, we also add 0. The reason for this
  // is that 0 is a turning point.
  if (allownegative)
    {
      VectorInsertGridPoints(abs_nls_pert, abs_nls_pert, MakeVector(0), verbosity);
      out2 << "  I am including also 0 in the abs_nls_pert, because it is a turning \n"
           << "  point. Consider to use a higher abs_nls_interp_order, for example 4.\n";
    }
 
  out2 << "  abs_nls_pert: " << abs_nls_pert[0] << " to " << abs_nls_pert[abs_nls_pert.nelem()-1]
       << " (fractional units) in steps of " << abs_nls_pert[1]-abs_nls_pert[0]
       << " (" << abs_nls_pert.nelem() << " grid points)\n";
 
}
 
 
/* Workspace method: Doxygen documentation will be auto-generated */
void abs_lookupSetup(// WS Output:
                     Vector& abs_p,
                     Vector& abs_t,
                     Vector& abs_t_pert,
                     Matrix& abs_vmrs,
                     ArrayOfArrayOfSpeciesTag& abs_nls,
                     Vector& abs_nls_pert,
                     // WS Input:
                     const Index& atmosphere_dim,
                     const Vector& p_grid,
                     const Vector& lat_grid,
                     const Vector& lon_grid,
                     const Tensor3& t_field,
                     const Tensor4& vmr_field,
                     const ArrayOfArrayOfSpeciesTag& abs_species,
                     const Index& abs_p_interp_order,
                     const Index& abs_t_interp_order,
                     const Index& abs_nls_interp_order,
                     // Control Parameters:
                     const Numeric& p_step10,
                     const Numeric& t_step,
                     const Numeric& h2o_step,
                     const Verbosity& verbosity)
{
  // For consistency with other code around arts (e.g., correlation
  // lengths in atmlab), p_step is given as log10(p[Pa]). However, we
  // convert it here to the natural log:
  const Numeric p_step = log(pow(10.0, p_step10));
 
  // Checks on input parameters:
  
  // We don't actually need lat_grid and lon_grid, but we have them as
  // input variables, so that we can use the standard functions to
  // check atmospheric fields and grids. A bit cheesy, but I don't
  // want to program all the checks explicitly.
 
  // Check grids:
  chk_atm_grids(atmosphere_dim, p_grid, lat_grid, lon_grid);
 
  if (p_grid.nelem()<2)
    {
      ostringstream os;
      os << "You need at least two pressure levels.";
      throw runtime_error( os.str() );
    }
 
  // Check T field:
  chk_atm_field("t_field", t_field, atmosphere_dim,
                p_grid, lat_grid, lon_grid);
 
  // Check VMR field (and abs_species):
  chk_atm_field("vmr_field", vmr_field, atmosphere_dim,
                abs_species.nelem(), p_grid, lat_grid, lon_grid);
 
  // Check the keyword arguments:
  if ( p_step <= 0 || t_step <= 0 || h2o_step <= 0 )
    {
      ostringstream os;
      os << "The keyword arguments p_step, t_step, and h2o_step must be >0.";
      throw runtime_error( os.str() );
    }
 
  // Ok, all input parameters seem to be reasonable.
 
 
  // We will need the log of the pressure grid:
  Vector log_p_grid(p_grid.nelem());
  transform(log_p_grid, log, p_grid);
 
  //  const Numeric epsilon = 0.01 * p_step; // This is the epsilon that
  //                                         // we use for comparing p grid spacings.
 
  // Construct abs_p
  // ---------------
 
  ArrayOfNumeric log_abs_p_a;  // We take log_abs_p_a as an array of
                             // Numeric, so that we can easily 
                             // build it up by appending new elements to the end. 
 
  // Check whether there are pressure levels that are further apart
  // (in log(p)) than p_step, and insert additional levels if
  // necessary:
 
  log_abs_p_a.push_back(log_p_grid[0]);
 
  for (Index i=1; i<log_p_grid.nelem(); ++i)
    {
      const Numeric dp =  log_p_grid[i-1] - log_p_grid[i]; // The grid is descending.
 
      const Numeric dp_by_p_step = dp/p_step;
      //          cout << "dp_by_p_step: " << dp_by_p_step << "\n";
 
      // How many times does p_step fit into dp?
      const Index n = (Index) ceil(dp_by_p_step); 
      // n is the number of intervals that we want to have in the
      // new grid. The number of additional points to insert is
      // n-1. But we have to insert the original point as well.
      //          cout << n << "\n";
 
      const Numeric ddp = dp/(Numeric)n;
      //          cout << "ddp: " << ddp << "\n";
 
      for (Index j=1; j<=n; ++j)
        log_abs_p_a.push_back(log_p_grid[i-1] - (Numeric)j*ddp);          
    }
 
  // Copy to a proper vector, we need this also later for
  // interpolation: 
  Vector log_abs_p(log_abs_p_a.nelem());
  for (Index i=0; i<log_abs_p_a.nelem(); ++i)
    log_abs_p[i] = log_abs_p_a[i];
 
  // Copy the new grid to abs_p, removing the log:
  abs_p.resize(log_abs_p.nelem());
  transform(abs_p, exp, log_abs_p);
 
  // Check that abs_p has enough points for the interpolation order
  if (abs_p.nelem()<abs_p_interp_order+1)
    {
      ostringstream os;
      os << "Your pressure grid does not have enough levels for the desired interpolation order.";
      throw runtime_error( os.str() );
    }
 
  // We will also have to interpolate T and VMR profiles to the new
  // pressure grid. We interpolate in log(p), as usual in ARTS.
 
  // Grid positions:
  ArrayOfGridPos gp(log_abs_p.nelem());
  gridpos(gp, log_p_grid, log_abs_p);
 
  // Interpolation weights:
  Matrix itw(gp.nelem(),2);
  interpweights(itw,gp);
 
 
  // In the 1D case the lookup table is just a lookup table in
  // pressure. We treat this simple case first.
  if (1==atmosphere_dim)
    {
      // Reference temperature,
      // interpolate abs_t from t_field:
      abs_t.resize(log_abs_p.nelem());
      interp(abs_t,
             itw,
             t_field(joker,0,0),
             gp);
 
      // Temperature perturbations:
      abs_t_pert.resize(0);
 
      // Reference VMR profiles,
      // interpolate abs_vmrs from vmr_field:
      abs_vmrs.resize(abs_species.nelem(), log_abs_p.nelem());
      for (Index i=0; i<abs_species.nelem(); ++i)
        interp(abs_vmrs(i,joker),
               itw,
               vmr_field(i,joker,0,0),
               gp);
 
      // Species for which H2O VMR is perturbed:
      abs_nls.resize(0);
 
      // H2O VMR perturbations:
      abs_nls_pert.resize(0);
    }
  else
    {
      // 2D or 3D case. We have to set up T and nonlinear species variations.
 
      // Make an intelligent choice for the nonlinear species.
      choose_abs_nls(abs_nls, abs_species, verbosity);
 
      // Now comes a part where we analyse the atmospheric fields.
      // We need to find the max, min, and mean profile for
      // temperature and VMRs.
      // We do this on the original p grid, not on the refined p
      // grid, to be more efficient.
 
      // Temperature:
      Vector tmin(p_grid.nelem());
      Vector tmax(p_grid.nelem());
      Vector tmean(p_grid.nelem()); 
 
      for (Index i=0; i<p_grid.nelem(); ++i)
        {
          tmin[i]  = min(t_field(i,joker,joker));
          tmax[i]  = max(t_field(i,joker,joker));
          tmean[i] = mean(t_field(i,joker,joker));
        }
      
//       cout << "Tmin: " << tmin << "\n";
//       cout << "Tmax: " << tmax << "\n";
//       cout << "Tmean: " << tmean << "\n";
 
      // Calculate mean profiles of all species. (We need all for abs_vmrs
      // not only H2O.)
      Matrix vmrmean(abs_species.nelem(), p_grid.nelem()); 
      for (Index s=0; s<abs_species.nelem(); ++s)
        for (Index i=0; i<p_grid.nelem(); ++i)
          {
            vmrmean(s,i) = mean(vmr_field(s,i,joker,joker));
          }      
 
      // If there are NLS, determine H2O statistics:
 
      // We have to define these here, outside the if block, because
      // we need the values later.
      Vector h2omin(p_grid.nelem());
      Vector h2omax(p_grid.nelem());
      const Index h2o_index 
        = find_first_species_tg( abs_species,
                                 species_index_from_species_name("H2O") );
      // We need this inside the if clauses for nonlinear species
      // treatment. The function returns "-1" if there is no H2O
      // species. There is a check for that in the next if block, with
      // an appropriate runtime error.
 
      if (0<abs_nls.nelem())
        {
          // Check if there really is a H2O species.
          if (h2o_index<0)
            {
              ostringstream os;
              os << "Some of your species require nonlinear treatment,\n"
                 << "but you have no H2O species.";
              throw runtime_error( os.str() );
            }
 
          for (Index i=0; i<p_grid.nelem(); ++i)
            {
              h2omin[i]  = min(vmr_field(h2o_index,i,joker,joker));
              h2omax[i]  = max(vmr_field(h2o_index,i,joker,joker));
            }
      
//           cout << "H2Omin: " << h2omin << "\n";
//           cout << "H2Omax: " << h2omax << "\n";
//           cout << "H2Omean: " << vmrmean(h2o_index,joker) << "\n";
 
        }
 
 
      // Interpolate in pressure, set abs_t, abs_vmr...
 
      // Reference temperature,
      // interpolate abs_t from tmean:
      abs_t.resize(log_abs_p.nelem());
      interp(abs_t,
             itw,
             tmean,
             gp);
 
      // Temperature perturbations:
      choose_abs_t_pert(abs_t_pert, tmean, tmin, tmax, t_step,
                        abs_p_interp_order, abs_t_interp_order,
                        verbosity);
//       cout << "abs_t_pert: " << abs_t_pert << "\n";
 
      // Reference VMR profiles,
      // interpolate abs_vmrs from vmrmean:
      abs_vmrs.resize(abs_species.nelem(), log_abs_p.nelem());
      for (Index i=0; i<abs_species.nelem(); ++i)
        interp(abs_vmrs(i,joker),
               itw,
               vmrmean(i,joker),
               gp);
 
      if (0<abs_nls.nelem())
        {
          // Construct abs_nls_pert:
          choose_abs_nls_pert(abs_nls_pert,
                              vmrmean(h2o_index, joker),
                              h2omin,
                              h2omax,
                              h2o_step,
                              abs_p_interp_order,
                              abs_nls_interp_order,
                              verbosity);
        }
      else
        {
          // Empty abs_nls_pert:
          abs_nls_pert.resize(0);
        }
//       cout << "abs_nls_pert: " << abs_nls_pert << "\n";
 
    }
}
 
 
/* Workspace method: Doxygen documentation will be auto-generated */
void abs_lookupSetupBatch(// WS Output:
                          Vector& abs_p,
                          Vector& abs_t,
                          Vector& abs_t_pert,
                          Matrix& abs_vmrs,
                          ArrayOfArrayOfSpeciesTag& abs_nls,
                          Vector& abs_nls_pert,
                          // WS Input:
                          const ArrayOfArrayOfSpeciesTag& abs_species,
                          const ArrayOfGriddedField4& batch_fields,
                          const Index& abs_p_interp_order,
                          const Index& abs_t_interp_order,
                          const Index& abs_nls_interp_order,
                          // Control Parameters:
                          const Numeric& p_step10,
                          const Numeric& t_step,
                          const Numeric& h2o_step,
                          const Vector&  extremes,
                          const Verbosity& verbosity)
{
  CREATE_OUT2
  CREATE_OUT3
  
  // For consistency with other code around arts (e.g., correlation
  // lengths in atmlab), p_step is given as log10(p[Pa]). However, we
  // convert it here to the natural log:
  const Numeric p_step = log(pow(10.0, p_step10));
 
  // Derive field, where abs_species profiles start
  // (in batch_fields, first 2 are T and z, then we have an unknown number of
  // part_species fields, followed by N fields corresponding to the N entries in
  // abs_species)
  const Index indoff = batch_fields[0].data.nbooks()-abs_species.nelem();
//  cout << "abs species start at column " << indoff << ", i.e.,\n"
//       << indoff-2 << " particle related columns are present\n";
 
  // Derive which abs_species is H2O (required for nonlinear species handling)
  // returns -1 if no H2O present
  const Index h2o_index = find_first_species_tg( abs_species,
                            species_index_from_species_name("H2O") );
//  cout << "The " << h2o_index+1 << ". species in abs_species is H2O\n";
//  cout << "That is, H2O is expected to be the " << indoff+h2o_index
//       << ". column of the atmospheric fields\n";
 
  // Check that the field names in batch_fields are good. (We check
  // only the first field in the batch.)  
  {
    ostringstream os;
    bool bail = false;
 
    const ArrayOfString field_names = batch_fields[0].get_string_grid(0);
 
    ArrayOfString species_names(abs_species.nelem());
    for (Index i=0; i<abs_species.nelem(); ++i)
      species_names[i] = get_species_name(abs_species[i]);
 
    // First simply check dimensions of abs_species and field_names
    if (field_names.nelem() < 3)
      {
        os << "Atmospheric states in batch_fields must have at\n"
           << "least three fields: T, z, and at least one species.";
        throw runtime_error( os.str() );
      }
 
    if (abs_species.nelem() < 1)
      {
        os << "At least one absorption species needs to be defined "
           << "in abs_species.";
        throw runtime_error( os.str() );
      }
 
/*
    if (abs_species.nelem() != field_names.nelem()-2)
      {
        os << "Dimension of fields in batch_fields does not match that of abs_species.\n"
           << "(First two fields must be T and z, rest must match absorption species.)\n"
           << "Field names:        " << field_names << "\n"
           << "Absorption species: " << species_names; 
        throw runtime_error( os.str() );
      }
*/
    // field_names.nelem = 2(T,z) + M(part_species) + N(abs_species)
    // but we don't know M, so we can only check whether we have at least
    // N+2 (abs_species+T+z) atmospheric fields
    if (abs_species.nelem() >  field_names.nelem()-2)
      {
        os << "Dimension of fields in batch_fields does not match that of abs_species.\n"
           << "(First two fields must be T and z, the N last fields must match the\n"
           << "absorption species, where N is the number of species as defined in abs_species.)\n"
           << "Field names:        " << field_names << "\n"
           << "Absorption species: " << species_names; 
        throw runtime_error( os.str() );
      }
 
    os << "The consistency check of the first compact atmospheric state in\n"
       << "batch_fields has shown one or more errors:\n";
 
    // First field must be T:
    if ("T" != field_names[0])
      {
        os << "The first field name must be T.\n";
        bail = true;
      }
 
    // Second field must be z:
    if ("z" != field_names[1])
      {
        os << "The second field name must be z.\n";
        bail = true;
      }
 
    // One of the abs_species has to be H2O
    // (ideally that should be checked later, when we know whether there are
    // nonlinear species present. however, currently batch mode REQUIRES H2O to
    // be present, so we check that immediately)
    if (h2o_index < 0)
      {
        os << "One of the atmospheric fields must be H2O.\n";
        bail = true;
      }
 
    // The other fields must match abs_species:
    bool wrong_species = false;
    for (Index i=0; i<abs_species.nelem(); ++i)
      if (field_names[indoff+i] != species_names[i])
        wrong_species = true;
 
    if (wrong_species)
      {
        os << "The " << abs_species.nelem() << " last field names must "
           << "match the absorption species in\n"
           << "abs_species, which are:\n"
           << species_names << "\n";
        bail = true;
      }
 
    os << "Your field names are:\n"
       << field_names;
 
    if (bail) throw runtime_error( os.str() );
  }
 
  // FIXME: Adjustment of min/max values for Jacobian perturbations is still missing. 
 
  // Make an intelligent choice for the nonlinear species.
  choose_abs_nls(abs_nls, abs_species, verbosity);
 
  // Find out maximum and minimum pressure.
  Numeric maxp=batch_fields[0].get_numeric_grid(GFIELD4_P_GRID)[0];
  Numeric minp=batch_fields[0].get_numeric_grid(GFIELD4_P_GRID)[batch_fields[0].get_numeric_grid(GFIELD4_P_GRID).nelem()-1];
  for (Index i=0; i<batch_fields.nelem(); ++i)
    {
      if (maxp < batch_fields[i].get_numeric_grid(GFIELD4_P_GRID)[0])
        maxp = batch_fields[i].get_numeric_grid(GFIELD4_P_GRID)[0];
      if (minp > batch_fields[i].get_numeric_grid(GFIELD4_P_GRID)[batch_fields[i].get_numeric_grid(GFIELD4_P_GRID).nelem()-1])
        minp = batch_fields[i].get_numeric_grid(GFIELD4_P_GRID)[batch_fields[i].get_numeric_grid(GFIELD4_P_GRID).nelem()-1];
    }
  //  cout << "  minp/maxp: " << minp << " / " << maxp << "\n";
 
  if (maxp==minp)
    {
      ostringstream os;
      os << "You need at least two pressure levels.";
      throw runtime_error( os.str() );
    }
 
  // We construct the pressure grid as follows:
  // - Everything is done in log(p).
  // - Start with maxp and go down in steps of p_step until we are <= minp.
  // - Adjust the final pressure value to be exactly minp, otherwise
  //   we have problems in getting min, max, and mean values for this
  //   point later.
  Index np = (Index) ceil((log(maxp)-log(minp))/p_step)+1;
  // The +1 above has to be there because we must include also both
  // grid end points. 
 
  // If np is too small for the interpolation order, we increase it:
  if (np < abs_p_interp_order+1)
    np = abs_p_interp_order+1;
 
  Vector log_abs_p(log(maxp), np, -p_step);
  log_abs_p[np-1] = log(minp);
 
  abs_p.resize(np);
  transform(abs_p, exp, log_abs_p);  
  out2 << "  abs_p: " << abs_p[0] << " Pa to " << abs_p[np-1]
       << " Pa in log10 steps of " << p_step10
       << " (" << np << " grid points)\n";
 
  // Now we have to determine the statistics of T and VMRs, we need
  // profiles of min, max, and mean of these, on the abs_p grid.
 
//  Index n_variables = batch_fields[0].data.nbooks();
  // we will do statistics for data fields excluding the part_species fields
  Index n_variables = 2+abs_species.nelem();
 
  // The first dimension of datamin, datamax, and datamean is the
  // variable (T,Z,H2O,O3,...). The second dimension is pressure. We
  // assume all elements of the batch have the same variables.
 
  Matrix datamin  (n_variables, np, numeric_limits<Numeric>::max());
  Matrix datamax  (n_variables, np, numeric_limits<Numeric>::min());
  // The limits here are from the header file <limits>
  Matrix datamean (n_variables, np, 0);
  Vector mean_norm(np,0);          // Divide by this to get mean.
 
 
  // We will loop over all batch cases to analyze the statistics and
  // calculate reference profiles. As a little side project, we will
  // also calculate the absolute min and max of temperature and
  // humidity. This is handy as input to abs_lookupSetupWide.
  Numeric mint=+1e99;
  Numeric maxt=-1e99;
  Numeric minh2o=+1e99;
  Numeric maxh2o=-1e99;
 
  for (Index i=0; i<batch_fields.nelem(); ++i)
    {
      // Check that really each case has the same variables (see
      // comment above.)
      if (batch_fields[i].get_string_grid(GFIELD4_FIELD_NAMES)
          != batch_fields[0].get_string_grid(GFIELD4_FIELD_NAMES))
        throw runtime_error("All fields in the batch must have the same field names.");
 
      // Check that the first field is indeed T 
      if ("T" != batch_fields[i].get_string_grid(GFIELD4_FIELD_NAMES)[0])
        {
          ostringstream os;
          os << "The first variable in the compact atmospheric state must be \"T\",\n"
             << "but yours is: " << batch_fields[i].get_string_grid(GFIELD4_FIELD_NAMES)[0];
          throw runtime_error( os.str() );
        }
 
      // Check that field h2o_index+indoff is indeed H2O:
      if ("H2O" != 
        batch_fields[i].get_string_grid(GFIELD4_FIELD_NAMES)[indoff+h2o_index])
        {
          ostringstream os;
          os << "According to abs_species setting the " << indoff+h2o_index
             << ". variable in the compact\n"
             << "atmospheric state is supposed to be \"H2O\",\n"
             << "but yours is: "
             << batch_fields[i].get_string_grid(GFIELD4_FIELD_NAMES)[indoff+h2o_index];
          throw runtime_error( os.str() );
        }
 
      // Create convenient handles:
      const Vector&  p_grid = batch_fields[i].get_numeric_grid(GFIELD4_P_GRID);
      const Tensor4& data   = batch_fields[i].data;
 
      // Update our global max/min values for T and H2O:
 
      // We have to loop over pressure, latitudes, and longitudes
      // here. The dimensions of data are:
      // data[N_fields][N_p][N_lat][N_lon]
 
      for (Index ip=0; ip<data.npages(); ++ip)
        for (Index ilat=0; ilat<data.nrows(); ++ilat)
          for (Index ilon=0; ilon<data.ncols(); ++ilon)
        {
          // Field 0 is temperature:
          if (data(0,ip,ilat,ilon) < mint) mint = data(0,ip,ilat,ilon);
          if (data(0,ip,ilat,ilon) > maxt) maxt = data(0,ip,ilat,ilon);
          // Field indoff+h2o_index is H2O:
          if (data(indoff+h2o_index,ip,ilat,ilon) < minh2o)
            { minh2o = data(indoff+h2o_index,ip,ilat,ilon); }
          if (data(indoff+h2o_index,ip,ilat,ilon) > maxh2o)
            { maxh2o = data(indoff+h2o_index,ip,ilat,ilon); }
        }
 
      // Interpolate the current batch fields to the abs_p grid. We
      // have to do this for each batch case, since the grids could
      // all be different.
      
      Vector log_p_grid(p_grid.nelem());
      transform(log_p_grid, log, p_grid);
 
      // There is a catch here: We can only use the part of abs_p that
      // is inside the current pressure grid p_grid, otherwise we
      // would have to extrapolate.
      // The eps_bottom and eps_top account for the little bit of
      // extrapolation that is allowed.  
 
      const Numeric eps_bottom = (log_p_grid[0]-log_p_grid[1])/2.1;
      Index first_p=0;
      while (log_abs_p[first_p] > log_p_grid[0]+eps_bottom) ++first_p;
 
      const Numeric eps_top = (log_p_grid[log_p_grid.nelem()-2]-log_p_grid[log_p_grid.nelem()-1])/2.1;
      Index last_p=log_abs_p.nelem()-1;
      while (log_abs_p[last_p] < log_p_grid[log_p_grid.nelem()-1]-eps_top) --last_p;
 
      const Index extent_p = last_p - first_p + 1;
      
      // This was too complicated to get right:
      //      const Index first_p   = (Index) round ( (log_abs_p[0]       - log_p_grid[0])                    / p_step);
      //      const Index extent_p  = (Index) round ( (log_abs_p[first_p] - log_p_grid[log_p_grid.nelem()-1]) / p_step) + 1;
 
      
      ConstVectorView active_log_abs_p = log_abs_p[Range(first_p, extent_p)];
 
//       cout << "first_p / last_p / extent_p : " << first_p << " / " << last_p << " / " << extent_p << "\n";
//       cout << "log_p_grid: "        << log_p_grid << "\n";
//       cout << "log_abs_p:  "        << log_abs_p << "\n";
//       cout << "active_log_abs_p:  " << active_log_abs_p << "\n";
//       cout << "=============================================================\n";
//       arts_exit();
 
      // Grid positions:
      ArrayOfGridPos gp(active_log_abs_p.nelem());
      gridpos(gp, log_p_grid, active_log_abs_p);
      //      gridpos(gp, log_p_grid, active_log_abs_p, 100);
      // We allow much more extrapolation here than normal (0.5 is
      // normal). If we do not do this, then we get problems for
      // p_grids that are much finer than abs_p.
 
      // Interpolation weights:
      Matrix itw(gp.nelem(),2);
      interpweights(itw,gp);
 
      // We have to loop over fields, latitudes, and longitudes here, doing the
      // pressure interpolation for all. The dimensions of data are: 
      // data[N_fields][N_p][N_lat][N_lon]
      // For data_interp we reduce data by the particle related fields, i.e.,
      // the ones in between the first two and the last N=abs_species.nelem().
//      Tensor4 data_interp(data.nbooks(),
      Tensor4 data_interp(n_variables,
                          active_log_abs_p.nelem(),
                          data.nrows(),
                          data.ncols());
 
      for (Index lo=0; lo<data.ncols(); ++lo)
        for (Index la=0; la<data.nrows(); ++la)
          {
//          for (Index fi=0; fi<data.nbooks(); ++fi)
            // we have to handle T/z and abs_species parts separately since they
            // can have part_species in between, which we want to ignore
            for (Index fi=0; fi<2; ++fi)
              interp(data_interp(fi, joker, la, lo),
                     itw,
                     data(fi, joker, la, lo),
                     gp);
            for (Index fi=indoff; fi<data.nbooks(); ++fi)
              interp(data_interp(fi-indoff+2, joker, la, lo),
                     itw,
                     data(fi, joker, la, lo),
                     gp);
          }
 
      // Now update our datamin, datamax, and datamean variables.
      // We need the min and max only for the T and H2O profile, 
      // not for others. But we need the mean for all. We are just
      // hopping over the case that we do not need below. This is not
      // very clean, but efficient. And it avoids handling all the
      // different cases explicitly.
      for (Index lo=0; lo<data_interp.ncols(); ++lo)
        for (Index la=0; la<data_interp.nrows(); ++la)
          {
            for (Index fi=0; fi<data_interp.nbooks(); ++fi)
              {
                if (1!=fi)      // We skip the z field, which we do not need
                  for (Index pr=0; pr<data_interp.npages(); ++pr)
                    {
                      // Min and max only needed for T and H2o
                      if (0==fi || (h2o_index+2)==fi) 
                        {
                          if (data_interp(fi, pr, la, lo) < datamin(fi, first_p+pr))
                            datamin(fi, first_p+pr) = data_interp(fi, pr, la, lo);
                          if (data_interp(fi, pr, la, lo) > datamax(fi, first_p+pr))
                            datamax(fi, first_p+pr) = data_interp(fi, pr, la, lo);
                        }
                      
                      datamean(fi, first_p+pr) += data_interp(fi, pr, la, lo);
                    }
              }
 
            // The mean_norm is actually a bit tricky. It depends on
            // pressure, since different numbers of cases contribute
            // to the mean for different pressures. At the very eges
            // of the grid, typically only a single case contributes.
 
            mean_norm[Range(first_p, extent_p)] += 1;
          }
    }  
 
  out2 << "  Global statistics:\n"
       << "  min(p)   / max(p)   [Pa]:  " << minp << " / " << maxp << "\n"
       << "  min(T)   / max(T)   [K]:   " << mint << " / " << maxt << "\n"
       << "  min(H2O) / max(H2O) [VMR]: " << minh2o << " / " << maxh2o << "\n";
 
 
  // Divide mean by mean_norm to get the mean:
  assert(np==mean_norm.nelem());
  for (Index fi=0; fi<datamean.nrows(); ++fi)
    if (1!=fi)      // We skip the z field, which we do not need
      for (Index pi=0; pi<np; ++pi)
        {
          // We do this in an explicit loop here, since we have to
          // check whether there really were data points to calculate
          // the mean at each level.
          if (0<mean_norm[pi])
            datamean(fi,pi) /= mean_norm[pi];
          else
            {
              ostringstream os;
              os << "No data at pressure level " << pi+1
                 << " of "<< np << " (" << abs_p[pi] << " hPa).";
              throw runtime_error( os.str() );
            }
        }
 
  // If we do higher order pressure interpolation, then we should
  // smooth the reference profiles with a boxcar filter of width
  // p_interp_order+1. Otherwise we get numerical problems if there
  // are any sharp spikes in the reference profiles. 
  assert(log_abs_p.nelem()==np);
  Matrix smooth_datamean(datamean.nrows(), datamean.ncols(), 0);
  for (Index i=0; i<np; ++i)
    {
      GridPosPoly gp;
      gridpos_poly(gp, log_abs_p, log_abs_p[i], abs_p_interp_order);
 
      // We do this in practice by using the indices returned by
      // gridpos_poly. We simply take a mean over all points that
      // would be used in the interpolation.
 
      for (Index fi=0; fi<datamean.nrows(); ++fi)
        if (1!=fi)      // We skip the z field, which we do not need
          {
            for (Index j=0; j<gp.idx.nelem(); ++j)
              {
                smooth_datamean(fi,i) += datamean(fi,gp.idx[j]);
              }
            smooth_datamean(fi,i) /= (Numeric)gp.idx.nelem();            
          }
//       cout << "H2O-raw / H2O-smooth: "
//            << datamean(h2o_index+2,i) << " / "
//            << smooth_datamean(h2o_index+2,i) << "\n";
    }
 
  // There is another complication: If the (smoothed) mean for the H2O
  // reference profile is 0, then we have to adjust both mean and max
  // value to a non-zero number, otherwise the reference profile will
  // be zero, and we will get numerical problems later on when we
  // divide by the reference value. So, we set it here to 1e-9.
  for (Index i=0; i<np; ++i)
    {
      // Assert that really H2O has index h2o_index+indoff in the VMR field list
      assert("H2O" == 
        batch_fields[0].get_string_grid(GFIELD4_FIELD_NAMES)[indoff+h2o_index]);
 
      // Find mean and max H2O for this level:
      Numeric& mean_h2o = smooth_datamean(h2o_index+2,i);
      Numeric& max_h2o  = datamax(h2o_index+2,i);
      if (mean_h2o <= 0)
        {
          mean_h2o = 1e-9;
          max_h2o  = 1e-9;
          out3 << "  H2O profile contained zero values, adjusted to 1e-9.\n";
        }
    }
 
  // Set abs_t:
  abs_t.resize(np);
  abs_t = smooth_datamean(0,joker);
  //   cout << "abs_t: " << abs_t << "\n\n";
  //   cout << "tmin:  " << datamin(0,joker) << "\n\n";
  //   cout << "tmax:  " << datamax(0,joker) << "\n";
  
  // Set abs_vmrs:
  assert(abs_species.nelem()==smooth_datamean.nrows()-2);
  abs_vmrs.resize(abs_species.nelem(), np);
  abs_vmrs = smooth_datamean(Range(2,abs_species.nelem()),joker);
  //  cout << "\n\nabs_vmrs: " << abs_vmrs << "\n\n";
  
  // Construct abs_t_pert:
  ConstVectorView tmin = datamin(0,joker);
  ConstVectorView tmax = datamax(0,joker);
  choose_abs_t_pert(abs_t_pert, abs_t, tmin, tmax, t_step, abs_p_interp_order, abs_t_interp_order,
                    verbosity);
  //  cout << "abs_t_pert: " << abs_t_pert << "\n";
 
  // Construct abs_nls_pert:
  ConstVectorView h2omin = datamin(h2o_index+2,joker);
  ConstVectorView h2omax = datamax(h2o_index+2,joker);
  choose_abs_nls_pert(abs_nls_pert, abs_vmrs(h2o_index,joker),
                      h2omin, h2omax, h2o_step,
                      abs_p_interp_order, abs_nls_interp_order,
                      verbosity);
  //  cout << "abs_nls_pert: " << abs_nls_pert << "\n";
 
  // Append the explicitly given user extreme values, if necessary:
  if (0!=extremes.nelem())
    {
      // There must be 4 values in this case: t_min, t_max, h2o_min, h2o_max
      if (4!=extremes.nelem())
        {
          ostringstream os;
          os << "There must be exactly 4 elements in extremes:\n"
             << "min(abs_t_pert), max(abs_t_pert), min(abs_nls_pert), max(abs_nls_pert)";
          throw runtime_error( os.str() );
        }
      
      // t_min:
      if (extremes[0] < abs_t_pert[0])
        {
          Vector dummy = abs_t_pert;
          abs_t_pert.resize(abs_t_pert.nelem()+1);
          abs_t_pert[0] = extremes[0];
          abs_t_pert[Range(1,dummy.nelem())] = dummy;
          out2 << "  Added min extreme value for abs_t_pert: "
               << abs_t_pert[0] << "\n";
        }
 
      // t_max:
      if (extremes[1] > abs_t_pert[abs_t_pert.nelem()-1])
        {
          Vector dummy = abs_t_pert;
          abs_t_pert.resize(abs_t_pert.nelem()+1);
          abs_t_pert[Range(0,dummy.nelem())] = dummy;
          abs_t_pert[abs_t_pert.nelem()-1] = extremes[1];
          out2 << "  Added max extreme value for abs_t_pert: "
               << abs_t_pert[abs_t_pert.nelem()-1] << "\n";
        }
 
      // nls_min:
      if (extremes[2] < abs_nls_pert[0])
        {
          Vector dummy = abs_nls_pert;
          abs_nls_pert.resize(abs_nls_pert.nelem()+1);
          abs_nls_pert[0] = extremes[2];
          abs_nls_pert[Range(1,dummy.nelem())] = dummy;
          out2 << "  Added min extreme value for abs_nls_pert: "
               << abs_nls_pert[0] << "\n";
        }
 
      // nls_max:
      if (extremes[3] > abs_nls_pert[abs_nls_pert.nelem()-1])
        {
          Vector dummy = abs_nls_pert;
          abs_nls_pert.resize(abs_nls_pert.nelem()+1);
          abs_nls_pert[Range(0,dummy.nelem())] = dummy;
          abs_nls_pert[abs_nls_pert.nelem()-1] = extremes[3];
          out2 << "  Added max extreme value for abs_nls_pert: "
               << abs_nls_pert[abs_nls_pert.nelem()-1] << "\n";
        }
    }
}
 
 
void abs_lookupSetupWide(// WS Output:
                         Vector& abs_p,
                         Vector& abs_t,
                         Vector& abs_t_pert,
                         Matrix& abs_vmrs,
                         ArrayOfArrayOfSpeciesTag& abs_nls,
                         Vector& abs_nls_pert,
                         // WS Input:
                         const ArrayOfArrayOfSpeciesTag& abs_species,
                         const Index& abs_p_interp_order,
                         const Index& abs_t_interp_order,
                         const Index& abs_nls_interp_order,
                         // Control Parameters:
                         const Numeric& p_min,
                         const Numeric& p_max,
                         const Numeric& p_step10,
                         const Numeric& t_min,
                         const Numeric& t_max,
                         const Numeric& h2o_min,
                         const Numeric& h2o_max,
                         const Verbosity& verbosity)
{
  CREATE_OUT2
  
  // For consistency with other code around arts (e.g., correlation
  // lengths in atmlab), p_step is given as log10(p[Pa]). However, we
  // convert it here to the natural log:
  const Numeric p_step = log(pow(10.0, p_step10));
 
  // Make an intelligent choice for the nonlinear species.
  choose_abs_nls(abs_nls, abs_species, verbosity);
 
  // 1. Fix pressure grid abs_p
  // --------------------------
  
  // We construct the pressure grid as follows:
  // - Everything is done in log(p).
  // - Start with p_max and go down in steps of p_step until we are <= p_min.
 
  Index np = (Index) ceil((log(p_max)-log(p_min))/p_step)+1;
  // The +1 above has to be there because we must include also both
  // grid end points. 
 
  // If np is too small for the interpolation order, we increase it:
  if (np < abs_p_interp_order+1)
    np = abs_p_interp_order+1;
 
  Vector log_abs_p(log(p_max), np, -p_step);
 
  abs_p.resize(np);
  transform(abs_p, exp, log_abs_p);  
  out2 << "  abs_p: " << abs_p[0] << " Pa to " << abs_p[np-1]
       << " Pa in log10 steps of " << p_step10
       << " (" << np << " grid points)\n";
 
 
  // 2. Fix reference temperature profile abs_t and temperature perturbations
  // ------------------------------------------------------------------------
 
  // We simply take a constant reference profile.
 
  Numeric t_ref = (t_min + t_max) / 2;
 
  abs_t.resize(np);
  abs_t = t_ref;                // Assign same value to all elements.
 
  // We have to make vectors out of t_min and t_max, so we can use
  // them in the choose_abs_t_pert function call:
  Vector min_prof(np), max_prof(np);
  min_prof = t_min;
  max_prof = t_max;
 
  // Chose temperature perturbations:
  choose_abs_t_pert(abs_t_pert,
                    abs_t, min_prof, max_prof, 20,
                    abs_p_interp_order, abs_t_interp_order,
                    verbosity);
 
 
  // 3. Fix reference H2O profile and abs_nls_pert
  // ---------------------------------------------
 
  // We take a constant reference profile of 1000ppm (=1e-3)
 
  Numeric h2o_ref = 1e-3;
 
  // We have to assign this value to all pressures of the H2O profile,
  // and 0 to all other profiles.
 
  // abs_vmrs has dimension [n_species, np].
  abs_vmrs.resize(abs_species.nelem(), np);
  abs_vmrs = 0;
  
  // We look for O2 and N2, and assign constant values to them.
  // The values are from Wallace&Hobbs, 2nd edition.
 
  const Index o2_index 
    = find_first_species_tg( abs_species,
                             species_index_from_species_name("O2") );
  if (o2_index>=0)
    {
      abs_vmrs(o2_index, joker) = 0.2095;
    }
 
  const Index n2_index 
    = find_first_species_tg( abs_species,
                             species_index_from_species_name("N2") );
  if (n2_index>=0)
    {
      abs_vmrs(n2_index, joker) = 0.7808;
    }
 
 
  // Which species is H2O?
  const Index h2o_index 
    = find_first_species_tg( abs_species,
                             species_index_from_species_name("H2O") );
 
  // The function returns "-1" if there is no H2O
  // species.
  if (0<abs_nls.nelem())
    {
      if (h2o_index<0)
        {
          ostringstream os;
          os << "Some of your species require nonlinear treatment,\n"
             << "but you have no H2O species.";
          throw runtime_error( os.str() );
        }
 
      // Assign constant reference value to all H2O levels:
      abs_vmrs(h2o_index, joker) = h2o_ref;
 
      // We have to make vectors out of h2o_min and h2o_max, so we can use
      // them in the choose_abs_nls_pert function call. 
      // We re-use the vectors min_prof and max_prof that we have
      // defined above.
      min_prof = h2o_min;
      max_prof = h2o_max;
 
      // Construct abs_nls_pert:
      choose_abs_nls_pert(abs_nls_pert,
                          abs_vmrs(h2o_index, joker),
                          min_prof,
                          max_prof,
                          1e99,
                          abs_p_interp_order,
                          abs_nls_interp_order,
                          verbosity);     
    }
  else
    {
      CREATE_OUT1
      out1 << "  WARNING:\n"
           << "  You have no species that require H2O variations.\n"
           << "  This case might work, but it has never been tested.\n"
           << "  Please test it, then remove this warning.\n";
    }
}
 
 
/* Workspace method: Doxygen documentation will be auto-generated */
void abs_speciesAdd(// WS Output:
                    ArrayOfArrayOfSpeciesTag& abs_species,
                    // Control Parameters:
                    const ArrayOfString& names,
                    const Verbosity& verbosity)
{
  CREATE_OUT3
  
  // Size of initial array
  Index n_gs = abs_species.nelem();
  
  // Temporary ArrayOfSpeciesTag
  ArrayOfSpeciesTag temp;
    
  // Each element of the array of Strings names defines one tag
  // group. Let's work through them one by one.
  for ( Index i=0; i<names.nelem(); ++i )
    {
      array_species_tag_from_string( temp, names[i] );  
      abs_species.push_back(temp);
    }
 
  // Print list of tag groups to the most verbose output stream:
  out3 << "  Added tag groups:";
  for ( Index i=n_gs; i<abs_species.nelem(); ++i )
    {
      out3 << "\n  " << i << ":";
      for ( Index s=0; s<abs_species[i].nelem(); ++s )
        {
          out3 << " " << abs_species[i][s].Name();
        }
    }
  out3 << '\n';
}
 
 
/* Workspace method: Doxygen documentation will be auto-generated */
void abs_speciesAdd2(// WS Output:
                     Workspace&                ws,
                     ArrayOfArrayOfSpeciesTag& abs_species,
                     ArrayOfRetrievalQuantity& jq,
                     Agenda&                   jacobian_agenda,
                     // WS Input:
                     const Index&              atmosphere_dim,
                     const Vector&             p_grid,
                     const Vector&             lat_grid,
                     const Vector&             lon_grid,
                     // WS Generic Input:
                     const Vector&             rq_p_grid,
                     const Vector&             rq_lat_grid,
                     const Vector&             rq_lon_grid,
                     // Control Parameters:
                     const String&             species,
                     const String&             method,
                     const String&             mode,
                     const Numeric&            dx,
                     const Verbosity&          verbosity)
{
  CREATE_OUT3
  
  // Add species to *abs_species*
  ArrayOfSpeciesTag tags;
  array_species_tag_from_string( tags, species );
  abs_species.push_back( tags );
 
  // Print list of added tag group to the most verbose output stream:
  out3 << "  Appended tag group:";
  out3 << "\n  " << abs_species.nelem()-1 << ":";
  for ( Index s=0; s<tags.nelem(); ++s )
  {
    out3 << " " << tags[s].Name();
  }
  out3 << '\n';
 
  // Do retrieval part
  jacobianAddAbsSpecies( ws, jq, jacobian_agenda, atmosphere_dim, 
                         p_grid, lat_grid, lon_grid, rq_p_grid, rq_lat_grid, 
                         rq_lon_grid, species, method, mode, dx,
                         verbosity);
}
 
 
/* Workspace method: Doxygen documentation will be auto-generated */
void abs_speciesInit(ArrayOfArrayOfSpeciesTag& abs_species,
                     const Verbosity&)
{
  abs_species.resize(0);
}
 
 
/* Workspace method: Doxygen documentation will be auto-generated */
void SpeciesSet(// WS Generic Output:
                ArrayOfArrayOfSpeciesTag& abs_species,
                // Control Parameters:
                const ArrayOfString& names,
                const Verbosity& verbosity)
{
  CREATE_OUT3
  
  abs_species.resize(names.nelem());
 
  //cout << "Names: " << names << "\n";
 
  // Each element of the array of Strings names defines one tag
  // group. Let's work through them one by one.
  for ( Index i=0; i<names.nelem(); ++i )
    {
      // This part has now been moved to array_species_tag_from_string.
      // Call this function.
      array_species_tag_from_string( abs_species[i], names[i] );  
    }
 
  // Print list of tag groups to the most verbose output stream:
  out3 << "  Defined tag groups: ";
  for ( Index i=0; i<abs_species.nelem(); ++i )
    {
      out3 << "\n  " << i << ":";
      for ( Index s=0; s<abs_species[i].nelem(); ++s )
        {
          out3 << " " << abs_species[i][s].Name();
        }
    }
  out3 << '\n';
}
 
 
/* Workspace method: Doxygen documentation will be auto-generated */
void abs_lookupAdapt( GasAbsLookup&                   abs_lookup,
                      Index&                          abs_lookup_is_adapted,
                      const ArrayOfArrayOfSpeciesTag& abs_species,
                      const Vector&                   f_grid,
                      const Verbosity&                verbosity)
{
  abs_lookup.Adapt( abs_species, f_grid, verbosity );
  abs_lookup_is_adapted = 1;
}
 
 
/* Workspace method: Doxygen documentation will be auto-generated */
void abs_scalar_gasExtractFromLookup( Matrix&             abs_scalar_gas,
                                      const GasAbsLookup& abs_lookup,
                                      const Index&        abs_lookup_is_adapted, 
                                      const Index&        abs_p_interp_order,
                                      const Index&        abs_t_interp_order,
                                      const Index&        abs_nls_interp_order,
                                      const Index&        f_index,
                                      const Numeric&      a_pressure,
                                      const Numeric&      a_temperature,
                                      const Vector&       a_vmr_list,
                                      const Numeric&      a_doppler,
                                      const Numeric&      extpolfac,
                                      const Verbosity&    verbosity)
{
  CREATE_OUT3
  
  // Check if the table has been adapted:
  if ( 1!=abs_lookup_is_adapted )
    throw runtime_error("Gas absorption lookup table must be adapted,\n"
                        "use method abs_lookupAdapt.");
 
  // The function we are going to call here is one of the few helper
  // functions that adjust the size of their output argument
  // automatically. 
  abs_lookup.Extract( abs_scalar_gas,
                      abs_p_interp_order,
                      abs_t_interp_order,
                      abs_nls_interp_order,
                      f_index,
                      a_pressure,
                      a_temperature,
                      a_vmr_list );
 
  // Apply Doppler shift, if necessary.
  if (0==a_doppler)
    {
      out3 << "  Doppler shift: None\n";
    }
  else
    {
      ostringstream os;
      os << "  Doppler shift: " << a_doppler << " Hz\n";
      out3 << os.str();
 
      // Get original lookup table frequency grid.
      const Vector& f_grid_original = abs_lookup.GetFgrid();      
      const Index n_f_grid = f_grid_original.nelem();
 
      // Apply f_index, if necessary.
      Index f_start, f_extent;
      if ( f_index < 0 )
        {
          // This means we have extracted for all frequencies.
 
          f_start  = 0;
          f_extent = n_f_grid;
        }
      else
        {
          // This means we have extracted only for one frequency.
 
          // Check that f_index is inside f_grid: 
          // (Only an assert here, since abs_lookup.Extract(), which
          // we have called above, does exactly the same but gives a
          // runtime error.)
          assert ( f_index < n_f_grid );
      
          f_start  = f_index;
          f_extent = 1;
        }
      const Range f_range(f_start,f_extent);
      ConstVectorView f_grid_original_view = f_grid_original[f_range];
 
      // Get shifted frequency grid.
      Vector f_shifted = f_grid_original_view;
      f_shifted -= a_doppler;
 
      // Check that the shifted grid is within extpolfac of the original grid.
      chk_interpolation_grids("Absorption from original frequency grid "
                              "to Doppler-shifted frequency grid",
                              f_grid_original_view,
                              f_shifted,
                              1,
                              extpolfac );
    
      // Grid positions.
      ArrayOfGridPos gp(f_shifted.nelem());
      gridpos(gp,f_grid_original_view,f_shifted,extpolfac);
 
      // Weights.
      Matrix itw(gp.nelem(),2);
      interpweights(itw,gp);
 
      // Do interpolation.
      Vector abs_interpolated(abs_scalar_gas.nrows()); 
      // The vector temporarily holds interpolated absorption for each gas species.
      for (Index i=0; i<abs_scalar_gas.ncols(); ++i)
        {
          interp(abs_interpolated, 
                 itw, 
                 abs_scalar_gas(Range(joker),i), 
                 gp);
          // Copy interpolated absorption to abs_scalar_gas.
          abs_scalar_gas(Range(joker),i) = abs_interpolated;
        }
 
    }
}
 
 
/* Workspace method: Doxygen documentation will be auto-generated */
void abs_fieldCalc(Workspace& ws,
                   // WS Output:
                   Tensor5& asg_field,
                   // WS Input:
                   const Agenda&  sga_agenda,
                   const Index&   f_index,
                   const Vector&  f_grid,
                   const Index&   atmosphere_dim,
                   const Vector&  p_grid,
                   const Vector&  lat_grid,
                   const Vector&  lon_grid,
                   const Tensor3& t_field,
                   const Tensor4& vmr_field,
                   // WS Generic Input:
                   const Vector&  doppler,
                   const Verbosity& verbosity)
{
  CREATE_OUT2
  CREATE_OUT3
  
  Matrix  asg;
  Vector  a_vmr_list;
 
  // Get the number of species from the leading dimension of vmr_field:
  const Index n_species = vmr_field.nbooks();
 
  // Number of frequencies:
  const Index n_frequencies = f_grid.nelem();
 
  // Number of pressure levels:
  const Index n_pressures = p_grid.nelem();
 
  // Number of latitude grid points (must be at least one):
  const Index n_latitudes = max( Index(1), lat_grid.nelem() );
 
  // Number of longitude grid points (must be at least one):
  const Index n_longitudes = max( Index(1), lon_grid.nelem() );
  
  // Check grids:
  chk_atm_grids( atmosphere_dim,
                 p_grid,
                 lat_grid,
                 lon_grid );
  
  // Check if t_field is ok:
  chk_atm_field( "t_field",
                 t_field,
                 atmosphere_dim,
                 p_grid,
                 lat_grid,
                 lon_grid );
 
  // Check if vmr_field is ok.
  // (Actually, we are not checking the first dimension, since
  // n_species has been set from this.)
  chk_atm_field( "vmr_field",
                 vmr_field,
                 atmosphere_dim,
                 n_species,
                 p_grid,
                 lat_grid,
                 lon_grid );
 
  // Check that doppler is empty or matches p_grid
  if (0!=doppler.nelem() && p_grid.nelem()!=doppler.nelem())
    {
      ostringstream os;
      os << "Variable doppler must either be empty, or match the dimension of "
         << "p_grid.";
      throw runtime_error( os.str() );
    }
  
  // We also set the start and extent for the frequency loop.
  Index f_extent;
 
  if ( f_index < 0 )
    {
      // This means we should extract for all frequencies.
 
      f_extent = n_frequencies;
    }
  else
    {
      // This means we should extract only for one frequency.
 
      // Make sure that f_index is inside f_grid:
      if ( f_index >= n_frequencies )
        {
          ostringstream os;
          os << "The frequency index f_index points to a frequency outside"
             << "the frequency grid. (f_index = " << f_index
             << ", n_frequencies = " << n_frequencies << ")";
          throw runtime_error( os.str() );
        }
 
      f_extent = 1;
    }
 
  // Resize output field.
  // The dimension in lat and lon must be at least one, even if these
  // grids are empty.
  out2 << "  Creating field with dimensions:\n"
       << "    " << n_species << "    gas species,\n"
       << "    " << f_extent << "     frequencies,\n"
       << "    " << n_pressures << "  pressures,\n"
       << "    " << n_latitudes << "  latitudes,\n"
       << "    " << n_longitudes << " longitudes.\n";
 
  asg_field.resize( n_species,
                    f_extent,
                    n_pressures,
                    n_latitudes,
                    n_longitudes );
 
 
  // We have to make a local copy of the Workspace and the agendas because
  // only non-reference types can be declared firstprivate in OpenMP
  Workspace l_ws (ws);
  Agenda l_sga_agenda (sga_agenda);
 
 
  // Now we have to loop all points in the atmosphere:
/*#pragma omp parallel for                                                 \
  if(!arts_omp_in_parallel() && n_pressures>=arts_omp_get_max_threads()) \
  default(none)                                                          \
  shared(out3, joker, f_extent, p_grid, t_field, vmr_field, f_index,     \
         asg_field)                                                      \
  firstprivate(l_ws, l_sga_agenda)                                       \
  private(asg, a_vmr_list) */
#pragma omp parallel for                                                 \
  if(!arts_omp_in_parallel() && n_pressures>=arts_omp_get_max_threads()) \
  firstprivate(l_ws, l_sga_agenda)                                       \
  private(asg, a_vmr_list) 
  for ( Index ipr=0; ipr<n_pressures; ++ipr )         // Pressure:  ipr
    {
      // The try block here is necessary to correctly handle
      // exceptions inside the parallel region. 
      try
        {
          Numeric a_pressure = p_grid[ipr];
          
          Numeric a_doppler = 0;
          if (0!=doppler.nelem()) a_doppler = doppler[ipr];
          
          {
            ostringstream os;
            os << "  p_grid[" << ipr << "] = " << a_pressure << "\n";
            out3 << os.str();
          }
 
          for ( Index ila=0; ila<n_latitudes; ++ila )   // Latitude:  ila
            for ( Index ilo=0; ilo<n_longitudes; ++ilo ) // Longitude: ilo
              {
                Numeric a_temperature = t_field( ipr, ila, ilo );
                a_vmr_list    = vmr_field( Range(joker),
                                           ipr, ila, ilo );
 
                // Execute agenda to calculate local absorption.
                // Agenda input:  f_index, a_pressure, a_temperature, a_vmr_list
                // Agenda output: asg
                abs_scalar_gas_agendaExecute(l_ws,
                                             asg,
                                             f_index, a_doppler, a_pressure,
                                             a_temperature, a_vmr_list,
                                             l_sga_agenda);
 
                // Verify, that the number of species in asg is
                // constistent with vmr_field:
                if ( n_species != asg.ncols() )
                  {
                    ostringstream os;
                    os << "The number of gas species in vmr_field is "
                       << n_species << ",\n"
                       << "but the number of species returned by the agenda is "
                       << asg.ncols() << ".";
                    throw runtime_error( os.str() );
                  }
 
                // Verify, that the number of frequencies in asg is
                // constistent with f_extent:
                if ( f_extent != asg.nrows() )
                  {
                    ostringstream os;
                    os << "The number of frequencies desired is "
                       << n_frequencies << ",\n"
                       << "but the number of frequencies returned by the agenda is "
                       << asg.nrows() << ".";
                    throw runtime_error( os.str() );
                  }
 
                // Store the result in output field.
                // We have to transpose asg, because the dimensions of the
                // two variables are:
                // asg_field: [ abs_species, f_grid, p_grid, lat_grid, lon_grid]
                // asg:       [ f_grid, abs_species ]
                asg_field( Range(joker),
                           Range(joker),
                           ipr, ila, ilo ) = transpose( asg );
            
              }
        }
      catch (runtime_error e)
        {
          CREATE_OUT0
          exit_or_rethrow(e.what(), out0);
        }
    }
}
 
 
/* Workspace method: Doxygen documentation will be auto-generated */
void f_gridFromGasAbsLookup(
                            Vector&             f_grid,
                            const GasAbsLookup& abs_lookup,
                            const Verbosity&)
{
  const Vector& lookup_f_grid = abs_lookup.GetFgrid();
  f_grid.resize(lookup_f_grid.nelem());
  f_grid = lookup_f_grid;
}
 
 
/* Workspace method: Doxygen documentation will be auto-generated */
void p_gridFromGasAbsLookup(Vector&             p_grid,
                            const GasAbsLookup& abs_lookup,
                            const Verbosity&)
{
  const Vector& lookup_p_grid = abs_lookup.GetPgrid();
  p_grid.resize(lookup_p_grid.nelem());
  p_grid = lookup_p_grid;
}
 
 
 
Numeric calc_lookup_error(// Parameters for lookup table:
                          const GasAbsLookup& al,
                          const Index&        abs_p_interp_order,  
                          const Index&        abs_t_interp_order,  
                          const Index&        abs_nls_interp_order,
                          const bool          ignore_errors,
                          // Parameters for LBL:
                          const Vector&                   abs_n2,
                          const ArrayOfArrayOfLineRecord& abs_lines_per_species,
                          const ArrayOfLineshapeSpec&     abs_lineshape,
                          const ArrayOfString&            abs_cont_names,
                          const ArrayOfString&            abs_cont_models,
                          const ArrayOfVector&            abs_cont_parameters,
                          // Parameters for both:
                          const Numeric&      local_p,
                          const Numeric&      local_t,
                          const Vector&       local_vmrs,
                          const Verbosity&    verbosity)
{
  // Allocate some matrices. I also tried allocating these (and the
  // vectors below) outside, but there was no significant speed
  // advantage. (I guess the LBL calculation is expensive enough to
  // make the extra time of allocation here insignificant.)
  Matrix sga_tab;       // Absorption, dimension [n_f_grid,n_species]:
  Matrix sga_lbl;
 
  // Do lookup table first:
 
  try            
    {
      // Absorption, dimension [n_f_grid,n_species]:
      // Output variable: sga_tab
      al.Extract(sga_tab,
                 abs_p_interp_order,
                 abs_t_interp_order,
                 abs_nls_interp_order,
                 -1,
                 local_p,
                 local_t,
                 local_vmrs );
    }
  catch (runtime_error x)
    {
      // If ignore_errors is set to true, then we mark this case for
      // skipping, and ignore the exceptions.
      // Otherwise, we re-throw the exception.
      if (ignore_errors)
        return -1;
      else
        throw runtime_error(x.what());
    }
 
 
  // Get number of frequencies. (We cannot do this earlier, since we
  // get it from the output of al.Extract.)
  const Index n_f = sga_tab.nrows();
  
  // Allocate some vectors with this dimension:
  Vector abs_tab(n_f);
  Vector abs_lbl(n_f);
  Vector abs_rel_diff(n_f);                  
 
  // Sum up for all species, to get total absorption:          
  for (Index i=0; i<n_f; ++i)
    abs_tab[i] = sga_tab(i,joker).sum();
 
 
  // Now get absorption line-by-line.
 
  abs_scalar_gasCalcLBL( sga_lbl, 
                         al.f_grid, 
                         al.species, 
                         abs_n2, 
                         abs_lines_per_species, 
                         abs_lineshape, 
                         abs_cont_names, 
                         abs_cont_models, 
                         abs_cont_parameters, 
                         -1,
                         local_p,
                         local_t,
                         local_vmrs,
                         0,
                         verbosity);
  // Last argument above is the Doppler shift (usually
  // rte_doppler). Should be zero in this case.
 
  // Sum up for all species, to get total absorption:          
  for (Index i=0; i<n_f; ++i)
    abs_lbl[i] = sga_lbl(i,joker).sum();
 
 
  // Ok. What we have to compare is abs_tab and abs_lbl.
 
  assert(abs_tab.nelem()==n_f);
  assert(abs_lbl.nelem()==n_f);
  assert(abs_rel_diff.nelem()==n_f);
  for (Index i=0; i<n_f; ++i)
    {
      // Absolute value of relative difference in percent:
      abs_rel_diff[i] = fabs( (abs_tab[i] - abs_lbl[i]) / abs_lbl[i] * 100); 
    }
 
  // Maximum of this:
  Numeric max_abs_rel_diff = max(abs_rel_diff);
 
  //          cout << "ma " << max_abs_rel_diff << "\n";
 
  return  max_abs_rel_diff;
 
}
 
 
/* Workspace method: Doxygen documentation will be auto-generated */
void abs_lookupTestAccuracy(// WS Input:
                            const GasAbsLookup&             abs_lookup,
                            const Index&                    abs_lookup_is_adapted, 
                            const Index&                    abs_p_interp_order,
                            const Index&                    abs_t_interp_order,
                            const Index&                    abs_nls_interp_order,
                            const Vector&                   abs_n2,
                            const ArrayOfArrayOfLineRecord& abs_lines_per_species,
                            const ArrayOfLineshapeSpec&     abs_lineshape,
                            const ArrayOfString&            abs_cont_names,
                            const ArrayOfString&            abs_cont_models,
                            const ArrayOfVector&            abs_cont_parameters,
                            const Verbosity&                verbosity)
{
  CREATE_OUT2
  
  const GasAbsLookup& al = abs_lookup;
 
  // Check if the table has been adapted:
  if ( 1!=abs_lookup_is_adapted )
    throw runtime_error("Gas absorption lookup table must be adapted,\n"
                        "use method abs_lookupAdapt.");
 
 
  // Some important sizes:
  const Index n_nls     = al.nonlinear_species.nelem();
  const Index n_species = al.species.nelem();
  //  const Index n_f       = al.f_grid.nelem();
  const Index n_p       = al.log_p_grid.nelem();
 
  if ( n_nls <= 0 )
    {
      ostringstream os;
      os << "This function currently works only with lookup tables\n"
         << "containing nonlinear species.";
      throw runtime_error( os.str() );
    }
 
  // If there are nonlinear species, then at least one species must be
  // H2O. We will use that to perturb in the case of nonlinear
  // species.
  Index h2o_index = -1;
  if (n_nls>0)
    {
      h2o_index = find_first_species_tg( al.species,
                                         species_index_from_species_name("H2O") );
 
      // This is a runtime error, even though it would be more logical
      // for it to be an assertion, since it is an internal check on
      // the table. The reason is that it is somewhat awkward to check
      // for this in other places.
      if ( h2o_index == -1 )
        {
          ostringstream os;
          os << "With nonlinear species, at least one species must be a H2O species.";
          throw runtime_error( os.str() );
        }
    }
 
 
  // Check temperature interpolation
 
  Vector inbet_t_pert(al.t_pert.nelem()-1);
  for (Index i=0; i<inbet_t_pert.nelem(); ++i)
    inbet_t_pert[i] = (al.t_pert[i]+al.t_pert[i+1])/2.0;
 
  // To store the temperature error, which we define as the maximum of
  // the absolute value of the relative difference between LBL and
  // lookup table, in percent.
  Numeric err_t = -999;
 
/*#pragma omp parallel for                        \
  if(!arts_omp_in_parallel())                   \
  default(none)                                 \
  shared(inbet_t_pert, joker, h2o_index, err_t, al, abs_cont_parameters, \
         abs_cont_models, abs_cont_names, abs_lineshape,                 \
         abs_lines_per_species, abs_n2, abs_nls_interp_order,            \
         abs_t_interp_order, abs_p_interp_order) */
#pragma omp parallel for                        \
  if(!arts_omp_in_parallel())
  for (Index pi=0; pi<n_p; ++pi)
    for (Index ti=0; ti<inbet_t_pert.nelem(); ++ti)
      {
        // Find local conditions:
 
        // Pressure:
        Numeric local_p = al.p_grid[pi];
 
        // Temperature:
        Numeric local_t = al.t_ref[pi] + inbet_t_pert[ti];
 
        // VMRs:
        Vector local_vmrs = al.vmrs_ref(joker, pi);
 
        // Watch out, the table probably does not have an absorption
        // value for exactly the reference H2O profile. We multiply
        // with the first perturbation.
        local_vmrs[h2o_index] *= al.nls_pert[0];
 
        Numeric max_abs_rel_diff =
          calc_lookup_error(// Parameters for lookup table:
                            al,
                            abs_p_interp_order,  
                            abs_t_interp_order,  
                            abs_nls_interp_order,
                            true,                       // ignore errors
                            // Parameters for LBL:
                            abs_n2,
                            abs_lines_per_species,
                            abs_lineshape,
                            abs_cont_names,
                            abs_cont_models,
                            abs_cont_parameters,
                            // Parameters for both:
                            local_p,
                            local_t,
                            local_vmrs,
                            verbosity);
 
        //          cout << "ma " << max_abs_rel_diff << "\n";
 
        //Critical directive here is necessary, because all threads
        //access the same variable.
#pragma omp critical
        {
          if (max_abs_rel_diff > err_t)
            err_t = max_abs_rel_diff;
        }
 
      } // end parallel for loop
 
 
 
  // Check H2O interpolation
 
  Vector inbet_nls_pert(al.nls_pert.nelem()-1);
  for (Index i=0; i<inbet_nls_pert.nelem(); ++i)
    inbet_nls_pert[i] = (al.nls_pert[i]+al.nls_pert[i+1])/2.0;
 
  // To store the H2O error, which we define as the maximum of
  // the absolute value of the relative difference between LBL and
  // lookup table, in percent.
  Numeric err_nls = -999;
 
/*#pragma omp parallel for                                               \
  if(!arts_omp_in_parallel())                                          \
  default(none)                                                        \
  shared(inbet_nls_pert, joker, h2o_index, err_nls, al,                \
         abs_cont_parameters, abs_cont_models, abs_cont_names,         \
         abs_lineshape, abs_lines_per_species, abs_n2,                 \
         abs_nls_interp_order, abs_t_interp_order, abs_p_interp_order) */
#pragma omp parallel for                                               \
  if(!arts_omp_in_parallel())
  for (Index pi=0; pi<n_p; ++pi)
    for (Index ni=0; ni<inbet_nls_pert.nelem(); ++ni)
      {
        // Find local conditions:
 
        // Pressure:
        Numeric local_p = al.p_grid[pi];
 
        // Temperature:
 
        // Watch out, the table probably does not have an absorption
        // value for exactly the reference temperature. We add
        // the first perturbation.
 
        Numeric local_t = al.t_ref[pi] + al.t_pert[0];
 
        // VMRs:
        Vector local_vmrs = al.vmrs_ref(joker, pi);
 
        // Now we have to modify the H2O VMR according to nls_pert:
        local_vmrs[h2o_index] *= inbet_nls_pert[ni];
 
        Numeric max_abs_rel_diff =
          calc_lookup_error(// Parameters for lookup table:
                            al,
                            abs_p_interp_order,  
                            abs_t_interp_order,  
                            abs_nls_interp_order,
                            true,                       // ignore errors
                            // Parameters for LBL:
                            abs_n2,
                            abs_lines_per_species,
                            abs_lineshape,
                            abs_cont_names,
                            abs_cont_models,
                            abs_cont_parameters,
                            // Parameters for both:
                            local_p,
                            local_t,
                            local_vmrs,
                            verbosity);
 
        //Critical directive here is necessary, because all threads
        //access the same variable.
#pragma omp critical
        {
          if (max_abs_rel_diff > err_nls)
            err_nls = max_abs_rel_diff;
        }            
 
      } // end parallel for loop
 
 
  // Check pressure interpolation
 
  // IMPORTANT: This does not test the pure pressure interpolation,
  // unless we have constant reference profiles for T and
  // H2O. Otherwise we have T and H2O interpolation mixed in.
 
  Vector inbet_p_grid(n_p-1);
  Vector inbet_t_ref(n_p-1);
  Matrix inbet_vmrs_ref(n_species, n_p-1);
  for (Index i=0; i<inbet_p_grid.nelem(); ++i)
    {
      inbet_p_grid[i] = exp((al.log_p_grid[i]+al.log_p_grid[i+1])/2.0);
      inbet_t_ref[i]  = (al.t_ref[i]+al.t_ref[i+1])/2.0;
      for (Index j=0; j<n_species; ++j)
        inbet_vmrs_ref(j,i) = (al.vmrs_ref(j,i)+al.vmrs_ref(j,i+1))/2.0;
    }
  
  // To store the interpolation error, which we define as the maximum of
  // the absolute value of the relative difference between LBL and
  // lookup table, in percent.
  Numeric err_p = -999;
 
/*#pragma omp parallel for                                               \
  if(!arts_omp_in_parallel())                                          \
  default(none)                                                        \
  shared(inbet_p_grid, inbet_t_ref, joker, inbet_vmrs_ref, h2o_index,  \
         err_p, al, abs_cont_parameters, abs_cont_models,              \
         abs_cont_names, abs_lineshape, abs_lines_per_species, abs_n2, \
         abs_nls_interp_order, abs_t_interp_order, abs_p_interp_order) */
#pragma omp parallel for                                               \
  if(!arts_omp_in_parallel())
  for (Index pi=0; pi<n_p-1; ++pi)
    {
      // Find local conditions:
 
      // Pressure:
      Numeric local_p = inbet_p_grid[pi];
 
      // Temperature:
 
      // Watch out, the table probably does not have an absorption
      // value for exactly the reference temperature. We add
      // the first perturbation.
 
      Numeric local_t = inbet_t_ref[pi] + al.t_pert[0];
 
      // VMRs:
      Vector local_vmrs = inbet_vmrs_ref(joker, pi);
 
      // Watch out, the table probably does not have an absorption
      // value for exactly the reference H2O profile. We multiply
      // with the first perturbation.
      local_vmrs[h2o_index] *= al.nls_pert[0];
 
      Numeric max_abs_rel_diff =
        calc_lookup_error(// Parameters for lookup table:
                          al,
                          abs_p_interp_order,  
                          abs_t_interp_order,  
                          abs_nls_interp_order,
                          true,                         // ignore errors
                          // Parameters for LBL:
                          abs_n2,
                          abs_lines_per_species,
                          abs_lineshape,
                          abs_cont_names,
                          abs_cont_models,
                          abs_cont_parameters,
                          // Parameters for both:
                          local_p,
                          local_t,
                          local_vmrs,
                          verbosity);
 
      //Critical directive here is necessary, because all threads
      //access the same variable.
#pragma omp critical
      {
        if (max_abs_rel_diff > err_p)
          err_p = max_abs_rel_diff;
      }
    }
 
 
  // Check total error
 
  // To store the interpolation error, which we define as the maximum of
  // the absolute value of the relative difference between LBL and
  // lookup table, in percent.
  Numeric err_tot = -999;
 
/*#pragma omp parallel for                                               \
  if(!arts_omp_in_parallel())                                          \
  default(none)                                                        \
  shared(inbet_t_pert, inbet_nls_pert, inbet_p_grid,                   \
         inbet_t_ref, joker, inbet_vmrs_ref, h2o_index, err_tot,       \
         abs_cont_parameters, abs_cont_models, abs_cont_names,         \
         abs_lineshape, abs_lines_per_species, abs_n2,                 \
         abs_nls_interp_order, abs_t_interp_order, abs_p_interp_order, \
         al) */
#pragma omp parallel for                                               \
  if(!arts_omp_in_parallel())
  for (Index pi=0; pi<n_p-1; ++pi)
    for (Index ti=0; ti<inbet_t_pert.nelem(); ++ti)
      for (Index ni=0; ni<inbet_nls_pert.nelem(); ++ni)
    {
      // Find local conditions:
 
      // Pressure:
      Numeric local_p = inbet_p_grid[pi];
 
      // Temperature:
      Numeric local_t = inbet_t_ref[pi] + inbet_t_pert[ti];
 
      // VMRs:
      Vector local_vmrs = inbet_vmrs_ref(joker, pi);
 
      // Multiply with perturbation.
      local_vmrs[h2o_index] *= inbet_nls_pert[ni];
 
      Numeric max_abs_rel_diff =
        calc_lookup_error(// Parameters for lookup table:
                          al,
                          abs_p_interp_order,  
                          abs_t_interp_order,  
                          abs_nls_interp_order,
                          true,                         // ignore errors
                          // Parameters for LBL:
                          abs_n2,
                          abs_lines_per_species,
                          abs_lineshape,
                          abs_cont_names,
                          abs_cont_models,
                          abs_cont_parameters,
                          // Parameters for both:
                          local_p,
                          local_t,
                          local_vmrs,
                          verbosity);
 
      //Critical directive here is necessary, because all threads
      //access the same variable.
#pragma omp critical
      {
        if (max_abs_rel_diff > err_tot)
          {
            err_tot = max_abs_rel_diff;
            
//             cout << "New max error: pi, ti, ni, err_tot:\n"
//                  << pi << ", " << ti << ", " << ni << ", " << err_tot << "\n";
          }
      }
    }
 
 
  out2 << "  Max. of absolute value of relative error in percent:\n"
       << "  Note: Unless you have constant reference profiles, the\n"
       << "  pressure interpolation error will have other errors mixed in.\n"
       << "  Temperature interpolation: " << err_t << "%\n"
       << "  H2O (NLS) interpolation:   " << err_nls << "%\n"
       << "  Pressure interpolation:    " << err_p << "%\n"
       << "  Total error:               " << err_tot << "%\n";
 
  // Check pressure interpolation
 
//   assert(p_grid.nelem()==log_p_grid.nelem()); // Make sure that log_p_grid is initialized.
//   Vector inbet_log_p_grid(log_p_grid.nelem()-1)
//   for (Index i=0; i<log_p_grid.nelem()-1; ++i)
//     {
//       inbet_log_p_grid[i] = (log_p_grid[i]+log_p_grid[i+1])/2.0;
//     }
 
//   for (Index pi=0; pi<inbet_log_p_grid.nelem(); ++pi)
//     {
//       for (Index pt=0; pt<)
//     }  
 
}
 
/* Workspace method: Doxygen documentation will be auto-generated */
void abs_lookupTestAccMC(// WS Input:
                         const GasAbsLookup&             abs_lookup,
                         const Index&                    abs_lookup_is_adapted, 
                         const Index&                    abs_p_interp_order,
                         const Index&                    abs_t_interp_order,
                         const Index&                    abs_nls_interp_order,
                         const Vector&                   abs_n2,
                         const ArrayOfArrayOfLineRecord& abs_lines_per_species,
                         const ArrayOfLineshapeSpec&     abs_lineshape,
                         const ArrayOfString&            abs_cont_names,
                         const ArrayOfString&            abs_cont_models,
                         const ArrayOfVector&            abs_cont_parameters,
                         const Index&                    mc_seed,
                         const Verbosity&                verbosity)
{
  CREATE_OUT2
  CREATE_OUT3
  
  const GasAbsLookup& al = abs_lookup;
  
  // Check if the table has been adapted:
  if ( 1!=abs_lookup_is_adapted )
    throw runtime_error("Gas absorption lookup table must be adapted,\n"
                        "use method abs_lookupAdapt.");
  
  
  // Some important sizes:
  const Index n_nls     = al.nonlinear_species.nelem();
  const Index n_species = al.species.nelem();
  
  if ( n_nls <= 0 )
  {
    ostringstream os;
    os << "This function currently works only with lookup tables\n"
    << "containing nonlinear species.";
    throw runtime_error( os.str() );
  }
  
  // If there are nonlinear species, then at least one species must be
  // H2O. We will use that to perturb in the case of nonlinear
  // species.
  Index h2o_index = -1;
  if (n_nls>0)
  {
    h2o_index = find_first_species_tg( al.species,
                                      species_index_from_species_name("H2O") );
    
    // This is a runtime error, even though it would be more logical
    // for it to be an assertion, since it is an internal check on
    // the table. The reason is that it is somewhat awkward to check
    // for this in other places.
    if ( h2o_index == -1 )
    {
      ostringstream os;
      os << "With nonlinear species, at least one species must be a H2O species.";
      throw runtime_error( os.str() );
    }
  }
  
  // How many MC cases to run between each convergence check. 
  // (It is important for parallelization that this is not too small.)
  const Index chunksize = 100;
  
  //Random Number generator:
  Rng rng;
  rng.seed(mc_seed, verbosity);
  // rng.draw() will draw a double from the uniform distribution [0,1).
  
  // (Log) Pressure range:
  const Numeric lp_max = al.log_p_grid[0];
  const Numeric lp_min = al.log_p_grid[al.log_p_grid.nelem()-1];
  
  // T perturbation range (additive):
  const Numeric dT_min = al.t_pert[0];
  const Numeric dT_max = al.t_pert[al.t_pert.nelem()-1];
  
  // H2O perturbation range (scaling):
  const Numeric dh2o_min = al.nls_pert[0];
  const Numeric dh2o_max = al.nls_pert[al.nls_pert.nelem()-1];
  
  // We are creating all random numbers for the chunk beforehand, to avoid the 
  // problem that random number generators in different threads would need 
  // different seeds to produce independent random numbers.
  // (I prefer this solution to the one of having the rng inside the threads, 
  // because it ensures that the result does not depend on the the number of CPUs.)
  Vector rand_lp(chunksize);
  Vector rand_dT(chunksize);
  Vector rand_dh2o(chunksize);
  
  // Store the errors for one chunk:
  Vector max_abs_rel_diff(chunksize);
  
  // Flag to break our MC calculation loop eventually
  bool keep_looping=true;
  
  // Total mean and standard deviation. (Is updated after each chunk.)
  Numeric total_mean;
  Numeric total_std;
  Index N_chunk = 0;
  while (keep_looping)
  {
    ++N_chunk;
    
    for (Index i=0; i<chunksize; ++i)
    {
      // A random pressure, temperature perturbation, and H2O perturbation, 
      // all with flat PDF between min and max:
      rand_lp[i]   = rng.draw()*(lp_max-lp_min) + lp_min;
      rand_dT[i]   = rng.draw()*(dT_max-dT_min) + dT_min;
      rand_dh2o[i] = rng.draw()*(dh2o_max-dh2o_min) + dh2o_min;
    }
    
    for (Index i=0; i<chunksize; ++i)
    {
      // The pressure we work with here:
      const Numeric this_lp = rand_lp[i];
      
      // Now we have to interpolate t_ref and vmrs_ref to this 
      // pressure, so that we can apply the dT and dh2o perturbations. 
      
      // Pressure grid positions:
      ArrayOfGridPosPoly pgp(1);
      gridpos_poly(pgp,
                   al.log_p_grid,
                   this_lp,
                   abs_p_interp_order );
      
      // Pressure interpolation weights:
      Vector pitw;
      pitw.resize(abs_p_interp_order+1);
      interpweights(pitw,pgp[0]);
      
      // Interpolated temperature:
      const Numeric this_t_ref = interp(pitw,
                                        al.t_ref,    
                                        pgp[0]);
      
      // Interpolated VMRs:
      Vector these_vmrs(n_species);
      for (Index j=0; j<n_species; ++j)
      {
        these_vmrs[j] = interp(pitw,
                               al.vmrs_ref(j,Range(joker)),    
                               pgp[0]);
      }
      
      // Now get the actual p, T and H2O values:
      const Numeric this_p   = exp(this_lp);
      const Numeric this_t   = this_t_ref + rand_dT[i];
      these_vmrs[h2o_index] *= rand_dh2o[i];
      
      //            cout << "p, T, H2O: " << this_p << ", " << this_t << ", " << these_vmrs[h2o_index] << "\n";
      
      // Get error between table and LBL calculation for these conditions:
      
      max_abs_rel_diff[i] = calc_lookup_error(// Parameters for lookup table:
                                              al,
                                              abs_p_interp_order,  
                                              abs_t_interp_order,  
                                              abs_nls_interp_order,
                                              true,                       // ignore errors
                                              // Parameters for LBL:
                                              abs_n2,
                                              abs_lines_per_species,
                                              abs_lineshape,
                                              abs_cont_names,
                                              abs_cont_models,
                                              abs_cont_parameters,
                                              // Parameters for both:
                                              this_p,
                                              this_t,  
                                              these_vmrs,
                                              verbosity);
      //            cout << "max_abs_rel_diff[" << i << "] = " << max_abs_rel_diff[i] << "\n";
      
    }
    
    // Calculate Mean of the last batch.
    
    // Total number of valid points in the chunk (not counting negative values, 
    // which result from failed calculations at the edges of the table.)
    Index N=0; 
    // Mean (initially sum of all values):
    Numeric mean = 0;
    for (Index i=0; i<chunksize; ++i)
    {
      const Numeric x = max_abs_rel_diff[i];
      if (x > 0) 
      {
        ++N;
        mean += x;
      }
      //            else 
      //              {
      //                cout << "Negative value ignored.\n";
      //              }
    }        
    // Calculate mean by dividing sum by number of valid points:
    mean = mean / (Numeric)N;
    
    // Now calculate standard deviation:
    
    // Variance (initially sum of squared differences)
    Numeric variance = 0;
    for (Index i=0; i<chunksize; ++i)
    {
      const Numeric x = max_abs_rel_diff[i];
      if (x > 0) 
      {
        variance += (x - mean) * (x - mean);
      }
    }        
    // Divide by N to really calculate variance:
    variance = variance / (Numeric)N;
    
    //        cout << "Mean = " << mean << " Std = " << std_dev << "\n";
    
    if (N_chunk==1)
    {
      total_mean = mean;
      total_std  = sqrt(variance);
    }
    else 
    {
      const Numeric old_mean = total_mean;
      
      // This formula assimilates the new chunk mean into the total mean:
      total_mean = (total_mean*((Numeric)(N_chunk-1)) + mean) / (Numeric)N_chunk;
      
      // Do the same for the standard deviation.
      // First get rid of the square root:
      total_std = total_std * total_std;
      // Now multiply with old normalisation:
      total_std *= (Numeric)(N_chunk-1);
      // Now add the new sigma
      total_std += variance;
      // Divide by the new normalisation:
      total_std /= (Numeric)N_chunk;
      // And finally take the square root:
      total_std = sqrt(total_std);
      
      // Stop the chunk loop if desired accuracy has been reached. 
      // We take 1% here, no point in trying to be more accurate!
      if (abs(total_mean-old_mean) < total_mean/100) keep_looping = false;
    }
    
    //        cout << "  Chunk " << N_chunk << ": Mean estimate = " << total_mean 
    //             << " Std estimate = " << total_std << "\n";
    
    out3 << "  Chunk " << N_chunk << ": Mean estimate = " << total_mean 
    << " Std estimate = " << total_std << "\n";
    
  } // End of "keep_looping" loop that runs over the chunks
  
  out2 << "  Mean relative error: " << total_mean << "%\n"
  << "  Standard deviation:  " << total_std << "%\n";
  
}