lammps/src/fix_temp_berendsen.cpp

/* ----------------------------------------------------------------------
   LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
   http://lammps.sandia.gov, Sandia National Laboratories
   Steve Plimpton, sjplimp@sandia.gov

   Copyright (2003) Sandia Corporation.  Under the terms of Contract
   DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
   certain rights in this software.  This software is distributed under
   the GNU General Public License.

   See the README file in the top-level LAMMPS directory.
------------------------------------------------------------------------- */

#include <cstring>
#include <cstdlib>
#include <cmath>
#include "fix_temp_berendsen.h"
#include "atom.h"
#include "force.h"
#include "comm.h"
#include "input.h"
#include "variable.h"
#include "group.h"
#include "update.h"
#include "modify.h"
#include "compute.h"
#include "error.h"

using namespace LAMMPS_NS;
using namespace FixConst;

enum{NOBIAS,BIAS};
enum{CONSTANT,EQUAL};

/* ---------------------------------------------------------------------- */

FixTempBerendsen::FixTempBerendsen(LAMMPS *lmp, int narg, char **arg) :
  Fix(lmp, narg, arg),
  tstr(NULL), id_temp(NULL), tflag(0)
{
  if (narg != 6) error->all(FLERR,"Illegal fix temp/berendsen command");

  // Berendsen thermostat should be applied every step

  dynamic_group_allow = 1;
  nevery = 1;
  scalar_flag = 1;
  global_freq = nevery;
  extscalar = 1;

  tstr = NULL;
  if (strstr(arg[3],"v_") == arg[3]) {
    int n = strlen(&arg[3][2]) + 1;
    tstr = new char[n];
    strcpy(tstr,&arg[3][2]);
    tstyle = EQUAL;
  } else {
    t_start = force->numeric(FLERR,arg[3]);
    t_target = t_start;
    tstyle = CONSTANT;
  }

  t_stop = force->numeric(FLERR,arg[4]);
  t_period = force->numeric(FLERR,arg[5]);

  // error checks

  if (t_period <= 0.0)
    error->all(FLERR,"Fix temp/berendsen period must be > 0.0");

  // create a new compute temp style
  // id = fix-ID + temp, compute group = fix group

  int n = strlen(id) + 6;
  id_temp = new char[n];
  strcpy(id_temp,id);
  strcat(id_temp,"_temp");

  char **newarg = new char*[3];
  newarg[0] = id_temp;
  newarg[1] = group->names[igroup];
  newarg[2] = (char *) "temp";
  modify->add_compute(3,newarg);
  delete [] newarg;
  tflag = 1;

  energy = 0;
}

/* ---------------------------------------------------------------------- */

FixTempBerendsen::~FixTempBerendsen()
{
  delete [] tstr;

  // delete temperature if fix created it

  if (tflag) modify->delete_compute(id_temp);
  delete [] id_temp;
}

/* ---------------------------------------------------------------------- */

int FixTempBerendsen::setmask()
{
  int mask = 0;
  mask |= END_OF_STEP;
  mask |= THERMO_ENERGY;
  return mask;
}

/* ---------------------------------------------------------------------- */

void FixTempBerendsen::init()
{
  // check variable

  if (tstr) {
    tvar = input->variable->find(tstr);
    if (tvar < 0)
      error->all(FLERR,"Variable name for fix temp/berendsen does not exist");
    if (input->variable->equalstyle(tvar)) tstyle = EQUAL;
    else error->all(FLERR,"Variable for fix temp/berendsen is invalid style");
  }

  int icompute = modify->find_compute(id_temp);
  if (icompute < 0)
    error->all(FLERR,"Temperature ID for fix temp/berendsen does not exist");
  temperature = modify->compute[icompute];

  if (modify->check_rigid_group_overlap(groupbit))
    error->warning(FLERR,"Cannot thermostat atoms in rigid bodies");

  if (temperature->tempbias) which = BIAS;
  else which = NOBIAS;
}

/* ---------------------------------------------------------------------- */

void FixTempBerendsen::end_of_step()
{
  double t_current = temperature->compute_scalar();
  double tdof = temperature->dof;

  // there is nothing to do, if there are no degrees of freedom

  if (tdof < 1) return;

  if (t_current == 0.0)
    error->all(FLERR,
               "Computed temperature for fix temp/berendsen cannot be 0.0");

  double delta = update->ntimestep - update->beginstep;
  if (delta != 0.0) delta /= update->endstep - update->beginstep;

  // set current t_target
  // if variable temp, evaluate variable, wrap with clear/add

  if (tstyle == CONSTANT)
    t_target = t_start + delta * (t_stop-t_start);
  else {
    modify->clearstep_compute();
    t_target = input->variable->compute_equal(tvar);
    if (t_target < 0.0)
      error->one(FLERR,
                 "Fix temp/berendsen variable returned negative temperature");
    modify->addstep_compute(update->ntimestep + nevery);
  }

  // rescale velocities by lamda
  // for BIAS:
  //   temperature is current, so do not need to re-compute
  //   OK to not test returned v = 0, since lamda is multiplied by v

  double lamda = sqrt(1.0 + update->dt/t_period*(t_target/t_current - 1.0));
  double efactor = 0.5 * force->boltz * tdof;
  energy += t_current * (1.0-lamda*lamda) * efactor;

  double **v = atom->v;
  int *mask = atom->mask;
  int nlocal = atom->nlocal;

  if (which == NOBIAS) {
    for (int i = 0; i < nlocal; i++) {
      if (mask[i] & groupbit) {
        v[i][0] *= lamda;
        v[i][1] *= lamda;
        v[i][2] *= lamda;
      }
    }
  } else {
    for (int i = 0; i < nlocal; i++) {
      if (mask[i] & groupbit) {
        temperature->remove_bias(i,v[i]);
        v[i][0] *= lamda;
        v[i][1] *= lamda;
        v[i][2] *= lamda;
        temperature->restore_bias(i,v[i]);
      }
    }
  }
}

/* ---------------------------------------------------------------------- */

int FixTempBerendsen::modify_param(int narg, char **arg)
{
  if (strcmp(arg[0],"temp") == 0) {
    if (narg < 2) error->all(FLERR,"Illegal fix_modify command");
    if (tflag) {
      modify->delete_compute(id_temp);
      tflag = 0;
    }
    delete [] id_temp;
    int n = strlen(arg[1]) + 1;
    id_temp = new char[n];
    strcpy(id_temp,arg[1]);

    int icompute = modify->find_compute(id_temp);
    if (icompute < 0)
      error->all(FLERR,"Could not find fix_modify temperature ID");
    temperature = modify->compute[icompute];

    if (temperature->tempflag == 0)
      error->all(FLERR,
                 "Fix_modify temperature ID does not compute temperature");
    if (temperature->igroup != igroup && comm->me == 0)
      error->warning(FLERR,"Group for fix_modify temp != fix group");
    return 2;
  }
  return 0;
}

/* ---------------------------------------------------------------------- */

void FixTempBerendsen::reset_target(double t_new)
{
  t_target = t_start = t_stop = t_new;
}

/* ---------------------------------------------------------------------- */

double FixTempBerendsen::compute_scalar()
{
  return energy;
}

/* ----------------------------------------------------------------------
   extract thermostat properties
------------------------------------------------------------------------- */

void *FixTempBerendsen::extract(const char *str, int &dim)
{
  dim=0;
  if (strcmp(str,"t_target") == 0) {
    return &t_target;
  }
  return NULL;
}