velocity.cpp
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Created: Tue, Jul 8, 09:41

velocity.cpp
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	/* ----------------------------------------------------------------------
	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 "lmptype.h"
	#include "mpi.h"
	#include "math.h"
	#include "stdio.h"
	#include "stdlib.h"
	#include "string.h"
	#include "velocity.h"
	#include "atom.h"
	#include "update.h"
	#include "domain.h"
	#include "lattice.h"
	#include "input.h"
	#include "variable.h"
	#include "force.h"
	#include "modify.h"
	#include "fix.h"
	#include "compute.h"
	#include "compute_temp.h"
	#include "random_park.h"
	#include "group.h"
	#include "comm.h"
	#include "memory.h"
	#include "error.h"

	using namespace LAMMPS_NS;

	enum{CREATE,SET,SCALE,RAMP,ZERO};
	enum{ALL,LOCAL,GEOM};
	enum{NONE,CONSTANT,EQUAL,ATOM};

	#define WARMUP 100
	#define SMALL 0.001

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

	Velocity::Velocity(LAMMPS *lmp) : Pointers(lmp) {}

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

	void Velocity::command(int narg, char **arg)
	{
	if (narg < 2) error->all(FLERR,"Illegal velocity command");

	if (domain->box_exist == 0)
	error->all(FLERR,"Velocity command before simulation box is defined");
	if (atom->natoms == 0)
	error->all(FLERR,"Velocity command with no atoms existing");

	// atom masses must all be set

	atom->check_mass();

	// identify group

	igroup = group->find(arg[0]);
	if (igroup == -1) error->all(FLERR,"Could not find velocity group ID");
	groupbit = group->bitmask[igroup];

	// identify style

	if (strcmp(arg[1],"create") == 0) style = CREATE;
	else if (strcmp(arg[1],"set") == 0) style = SET;
	else if (strcmp(arg[1],"scale") == 0) style = SCALE;
	else if (strcmp(arg[1],"ramp") == 0) style = RAMP;
	else if (strcmp(arg[1],"zero") == 0) style = ZERO;
	else error->all(FLERR,"Illegal velocity command");

	// set defaults

	temperature = NULL;
	dist_flag = 0;
	sum_flag = 0;
	momentum_flag = 1;
	rotation_flag = 0;
	loop_flag = ALL;
	scale_flag = 1;
	rfix = -1;

	// read options from end of input line
	// change defaults as options specify

	if (style == CREATE) options(narg-4,&arg[4]);
	else if (style == SET) options(narg-5,&arg[5]);
	else if (style == SCALE) options(narg-3,&arg[3]);
	else if (style == RAMP) options(narg-8,&arg[8]);
	else if (style == ZERO) options(narg-3,&arg[3]);

	// initialize velocities based on style
	// create() invoked differently, so can be called externally

	if (style == CREATE) {
	double t_desired = force->numeric(FLERR,arg[2]);
	int seed = force->inumeric(FLERR,arg[3]);
	create(t_desired,seed);
	}
	else if (style == SET) set(narg-2,&arg[2]);
	else if (style == SCALE) scale(narg-2,&arg[2]);
	else if (style == RAMP) ramp(narg-2,&arg[2]);
	else if (style == ZERO) zero(narg-2,&arg[2]);
	}

	/* ----------------------------------------------------------------------
	initialization of defaults before calling velocity methods externaly
	------------------------------------------------------------------------- */

	void Velocity::init_external(const char *extgroup)
	{
	igroup = group->find(extgroup);
	if (igroup == -1) error->all(FLERR,"Could not find velocity group ID");
	groupbit = group->bitmask[igroup];

	temperature = NULL;
	dist_flag = 0;
	sum_flag = 0;
	momentum_flag = 1;
	rotation_flag = 0;
	loop_flag = ALL;
	scale_flag = 1;
	}

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

	void Velocity::create(double t_desired, int seed)
	{
	int i;

	if (seed <= 0) error->all(FLERR,"Illegal velocity create command");

	// if temperature = NULL, create a new ComputeTemp with the velocity group

	int tflag = 0;
	if (temperature == NULL) {
	char *arg = new char[3];
	arg[0] = (char *) "velocity_temp";
	arg[1] = group->names[igroup];
	arg[2] = (char *) "temp";
	temperature = new ComputeTemp(lmp,3,arg);
	tflag = 1;
	delete [] arg;
	}

	// initialize temperature computation
	// warn if groups don't match

	if (igroup != temperature->igroup && comm->me == 0)
	error->warning(FLERR,"Mismatch between velocity and compute groups");
	temperature->init();
	temperature->setup();

	// store a copy of current velocities

	double **v = atom->v;
	int nlocal = atom->nlocal;
	double **vhold;
	memory->create(vhold,nlocal,3,"velocity:vnew");

	for (i = 0; i < nlocal; i++) {
	vhold[i][0] = v[i][0];
	vhold[i][1] = v[i][1];
	vhold[i][2] = v[i][2];
	}

	// create new velocities, in uniform or gaussian distribution
	// loop option determines looping style, ALL is default
	// ALL = loop over all natoms, only set those I own via atom->map
	// cannot do this if atom IDs do not span 1-Natoms (some were deleted)
	// will produce same V, independent of P, if atoms were read-in
	// will NOT produce same V, independent of P, if used create_atoms
	// LOCAL = only loop over my atoms, adjust RNG to be proc-specific
	// will never produce same V, independent of P
	// GEOM = only loop over my atoms
	// choose RNG for each atom based on its xyz coord (geometry)
	// via random->reset()
	// will always produce same V, independent of P
	// adjust by factor for atom mass
	// for 2d, set Vz to 0.0

	double *rmass = atom->rmass;
	double *mass = atom->mass;
	int *type = atom->type;
	int *mask = atom->mask;
	int dimension = domain->dimension;

	int m;
	double vx,vy,vz,factor;
	RanPark *random;

	if (loop_flag == ALL) {

	// create an atom map if one doesn't exist already

	int mapflag = 0;
	if (atom->map_style == 0) {
	mapflag = 1;
	atom->map_style = 1;
	atom->nghost = 0;
	atom->map_init();
	atom->map_set();
	}

	// error check

	if (atom->natoms > MAXSMALLINT)
	error->all(FLERR,"Too big a problem to use velocity create loop all");
	if (atom->tag_enable == 0)
	error->all(FLERR,
	"Cannot use velocity create loop all unless atoms have IDs");
	if (atom->tag_consecutive() == 0)
	error->all(FLERR,
	"Atom IDs must be consecutive for velocity create loop all");

	// loop over all atoms in system
	// generate RNGs for all atoms, only assign to ones I own
	// use either per-type mass or per-atom rmass

	random = new RanPark(lmp,seed);
	int natoms = static_cast<int> (atom->natoms);

	for (i = 1; i <= natoms; i++) {
	if (dist_flag == 0) {
	vx = random->uniform();
	vy = random->uniform();
	vz = random->uniform();
	} else {
	vx = random->gaussian();
	vy = random->gaussian();
	vz = random->gaussian();
	}
	m = atom->map(i);
	if (m >= 0 && m < nlocal) {
	if (mask[m] & groupbit) {
	if (rmass) factor = 1.0/sqrt(rmass[m]);
	else factor = 1.0/sqrt(mass[type[m]]);
	v[m][0] = vx * factor;
	v[m][1] = vy * factor;
	if (dimension == 3) v[m][2] = vz * factor;
	else v[m][2] = 0.0;
	}
	}
	}

	// delete temporary atom map

	if (mapflag) {
	atom->map_delete();
	atom->map_style = 0;
	}

	} else if (loop_flag == LOCAL) {
	random = new RanPark(lmp,seed + comm->me);
	for (i = 0; i < WARMUP; i++) random->uniform();

	for (i = 0; i < nlocal; i++) {
	if (mask[i] & groupbit) {
	if (dist_flag == 0) {
	vx = random->uniform();
	vy = random->uniform();
	vz = random->uniform();
	} else {
	vx = random->gaussian();
	vy = random->gaussian();
	vz = random->gaussian();
	}
	if (rmass) factor = 1.0/sqrt(rmass[i]);
	else factor = 1.0/sqrt(mass[type[i]]);
	v[i][0] = vx * factor;
	v[i][1] = vy * factor;
	if (dimension == 3) v[i][2] = vz * factor;
	else v[i][2] = 0.0;
	}
	}

	} else if (loop_flag == GEOM) {
	random = new RanPark(lmp,1);
	double **x = atom->x;

	for (i = 0; i < nlocal; i++) {
	if (mask[i] & groupbit) {
	random->reset(seed,x[i]);
	if (dist_flag == 0) {
	vx = random->uniform();
	vy = random->uniform();
	vz = random->uniform();
	} else {
	vx = random->gaussian();
	vy = random->gaussian();
	vz = random->gaussian();
	}

	if (rmass) factor = 1.0/sqrt(rmass[i]);
	else factor = 1.0/sqrt(mass[type[i]]);
	v[i][0] = vx * factor;
	v[i][1] = vy * factor;
	if (dimension == 3) v[i][2] = vz * factor;
	else v[i][2] = 0.0;
	}
	}
	}

	// apply momentum and rotation zeroing

	if (momentum_flag) zero_momentum();
	if (rotation_flag) zero_rotation();

	// scale temp to desired value

	double t = temperature->compute_scalar();
	rescale(t,t_desired);

	// if sum_flag set, add back in previous velocities

	if (sum_flag) {
	for (i = 0; i < nlocal; i++) {
	if (mask[i] & groupbit) {
	v[i][0] += vhold[i][0];
	v[i][1] += vhold[i][1];
	v[i][2] += vhold[i][2];
	}
	}
	}

	// free local memory
	// if temperature was created, delete it

	memory->destroy(vhold);
	delete random;
	if (tflag) delete temperature;
	}

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

	void Velocity::set(int narg, char **arg)
	{
	int xstyle,ystyle,zstyle,varflag;
	double vx,vy,vz;
	char xstr,ystr,*zstr;
	int xvar,yvar,zvar;

	// parse 3 args

	xstyle = ystyle = zstyle = CONSTANT;
	xstr = ystr = zstr = NULL;

	if (strstr(arg[0],"v_") == arg[0]) {
	int n = strlen(&arg[0][2]) + 1;
	xstr = new char[n];
	strcpy(xstr,&arg[0][2]);
	} else if (strcmp(arg[0],"NULL") == 0) xstyle = NONE;
	else vx = force->numeric(FLERR,arg[0]);

	if (strstr(arg[1],"v_") == arg[1]) {
	int n = strlen(&arg[1][2]) + 1;
	ystr = new char[n];
	strcpy(ystr,&arg[1][2]);
	} else if (strcmp(arg[1],"NULL") == 0) ystyle = NONE;
	else vy = force->numeric(FLERR,arg[1]);

	if (strstr(arg[2],"v_") == arg[2]) {
	int n = strlen(&arg[2][2]) + 1;
	zstr = new char[n];
	strcpy(zstr,&arg[2][2]);
	} else if (strcmp(arg[2],"NULL") == 0) zstyle = NONE;
	else vz = force->numeric(FLERR,arg[2]);

	// set and apply scale factors

	xscale = yscale = zscale = 1.0;

	if (xstyle && !xstr) {
	if (scale_flag) xscale = domain->lattice->xlattice;
	vx *= xscale;
	}
	if (ystyle && !ystr) {
	if (scale_flag) yscale = domain->lattice->ylattice;
	vy *= yscale;
	}
	if (zstyle && !zstr) {
	if (scale_flag) zscale = domain->lattice->zlattice;
	vz *= zscale;
	}

	// check variables

	if (xstr) {
	xvar = input->variable->find(xstr);
	if (xvar < 0)
	error->all(FLERR,"Variable name for velocity set does not exist");
	if (input->variable->equalstyle(xvar)) xstyle = EQUAL;
	else if (input->variable->atomstyle(xvar)) xstyle = ATOM;
	else error->all(FLERR,"Variable for velocity set is invalid style");
	}
	if (ystr) {
	yvar = input->variable->find(ystr);
	if (yvar < 0)
	error->all(FLERR,"Variable name for velocity set does not exist");
	if (input->variable->equalstyle(yvar)) ystyle = EQUAL;
	else if (input->variable->atomstyle(yvar)) ystyle = ATOM;
	else error->all(FLERR,"Variable for velocity set is invalid style");
	}
	if (zstr) {
	zvar = input->variable->find(zstr);
	if (zvar < 0)
	error->all(FLERR,"Variable name for velocity set does not exist");
	if (input->variable->equalstyle(zvar)) zstyle = EQUAL;
	else if (input->variable->atomstyle(zvar)) zstyle = ATOM;
	else error->all(FLERR,"Variable for velocity set is invalid style");
	}

	if (xstyle == ATOM \|\| ystyle == ATOM \|\| zstyle == ATOM)
	varflag = ATOM;
	else if (xstyle == EQUAL \|\| ystyle == EQUAL \|\| zstyle == EQUAL)
	varflag = EQUAL;
	else varflag = CONSTANT;

	// error check for 2d models

	if (domain->dimension == 2) {
	if (zstyle == CONSTANT && vz != 0.0)
	error->all(FLERR,"Cannot set non-zero z velocity for 2d simulation");
	if (zstyle == EQUAL \|\| zstyle == ATOM)
	error->all(FLERR,"Cannot set variable z velocity for 2d simulation");
	}

	// allocate vfield array if necessary

	double **vfield = NULL;
	if (varflag == ATOM) memory->create(vfield,atom->nlocal,3,"velocity:vfield");

	// set velocities via constants

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

	if (varflag == CONSTANT) {
	for (int i = 0; i < nlocal; i++) {
	if (mask[i] & groupbit) {
	if (sum_flag == 0) {
	if (xstyle) v[i][0] = vx;
	if (ystyle) v[i][1] = vy;
	if (zstyle) v[i][2] = vz;
	} else {
	if (xstyle) v[i][0] += vx;
	if (ystyle) v[i][1] += vy;
	if (zstyle) v[i][2] += vz;
	}
	}
	}

	// set velocities via variables

	} else {
	if (xstyle == EQUAL) vx = input->variable->compute_equal(xvar);
	else if (xstyle == ATOM && vfield)
	input->variable->compute_atom(xvar,igroup,&vfield[0][0],3,0);
	if (ystyle == EQUAL) vy = input->variable->compute_equal(yvar);
	else if (ystyle == ATOM && vfield)
	input->variable->compute_atom(yvar,igroup,&vfield[0][1],3,0);
	if (zstyle == EQUAL) vz = input->variable->compute_equal(zvar);
	else if (zstyle == ATOM && vfield)
	input->variable->compute_atom(zvar,igroup,&vfield[0][2],3,0);

	for (int i = 0; i < nlocal; i++)
	if (mask[i] & groupbit) {
	if (sum_flag == 0) {
	if (xstyle == ATOM) v[i][0] = vfield[i][0];
	else if (xstyle) v[i][0] = vx;
	if (ystyle == ATOM) v[i][1] = vfield[i][1];
	else if (ystyle) v[i][1] = vy;
	if (zstyle == ATOM) v[i][2] = vfield[i][2];
	else if (zstyle) v[i][2] = vz;
	} else {
	if (xstyle == ATOM) v[i][0] += vfield[i][0];
	else if (xstyle) v[i][0] += vx;
	if (ystyle == ATOM) v[i][1] += vfield[i][1];
	else if (ystyle) v[i][1] += vy;
	if (zstyle == ATOM) v[i][2] += vfield[i][2];
	else if (zstyle) v[i][2] += vz;
	}
	}
	}

	// clean up

	delete [] xstr;
	delete [] ystr;
	delete [] zstr;
	memory->destroy(vfield);
	}

	/* ----------------------------------------------------------------------
	rescale velocities of a group after computing its temperature
	------------------------------------------------------------------------- */

	void Velocity::scale(int narg, char **arg)
	{
	double t_desired = force->numeric(FLERR,arg[0]);

	// if temperature = NULL, create a new ComputeTemp with the velocity group

	int tflag = 0;
	if (temperature == NULL) {
	char *arg = new char[3];
	arg[0] = (char *) "velocity_temp";
	arg[1] = group->names[igroup];
	arg[2] = (char *) "temp";
	temperature = new ComputeTemp(lmp,3,arg);
	tflag = 1;
	delete [] arg;
	}

	// initialize temperature computation
	// warn if groups don't match

	if (igroup != temperature->igroup && comm->me == 0)
	error->warning(FLERR,"Mismatch between velocity and compute groups");
	temperature->init();
	temperature->setup();

	// scale temp to desired value

	double t = temperature->compute_scalar();
	rescale(t,t_desired);

	// if temperature was created, delete it

	if (tflag) delete temperature;
	}

	/* ----------------------------------------------------------------------
	apply a ramped set of velocities
	------------------------------------------------------------------------- */

	void Velocity::ramp(int narg, char **arg)
	{
	// set scale factors

	if (scale_flag) {
	xscale = domain->lattice->xlattice;
	yscale = domain->lattice->ylattice;
	zscale = domain->lattice->zlattice;
	}
	else xscale = yscale = zscale = 1.0;

	// parse args

	int v_dim;
	if (strcmp(arg[0],"vx") == 0) v_dim = 0;
	else if (strcmp(arg[0],"vy") == 0) v_dim = 1;
	else if (strcmp(arg[0],"vz") == 0) v_dim = 2;
	else error->all(FLERR,"Illegal velocity command");

	if (v_dim == 2 && domain->dimension == 2)
	error->all(FLERR,"Velocity ramp in z for a 2d problem");

	double v_lo,v_hi;
	if (v_dim == 0) {
	v_lo = xscale*force->numeric(FLERR,arg[1]);
	v_hi = xscale*force->numeric(FLERR,arg[2]);
	} else if (v_dim == 1) {
	v_lo = yscale*force->numeric(FLERR,arg[1]);
	v_hi = yscale*force->numeric(FLERR,arg[2]);
	} else if (v_dim == 2) {
	v_lo = zscale*force->numeric(FLERR,arg[1]);
	v_hi = zscale*force->numeric(FLERR,arg[2]);
	}

	int coord_dim;
	if (strcmp(arg[3],"x") == 0) coord_dim = 0;
	else if (strcmp(arg[3],"y") == 0) coord_dim = 1;
	else if (strcmp(arg[3],"z") == 0) coord_dim = 2;
	else error->all(FLERR,"Illegal velocity command");

	double coord_lo,coord_hi;
	if (coord_dim == 0) {
	coord_lo = xscale*force->numeric(FLERR,arg[4]);
	coord_hi = xscale*force->numeric(FLERR,arg[5]);
	} else if (coord_dim == 1) {
	coord_lo = yscale*force->numeric(FLERR,arg[4]);
	coord_hi = yscale*force->numeric(FLERR,arg[5]);
	} else if (coord_dim == 2) {
	coord_lo = zscale*force->numeric(FLERR,arg[4]);
	coord_hi = zscale*force->numeric(FLERR,arg[5]);
	}

	// vramp = ramped velocity component for v_dim
	// add or set based on sum_flag

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

	double fraction,vramp;

	for (int i = 0; i < nlocal; i++)
	if (mask[i] & groupbit) {
	fraction = (x[i][coord_dim] - coord_lo) / (coord_hi - coord_lo);
	fraction = MAX(fraction,0.0);
	fraction = MIN(fraction,1.0);
	vramp = v_lo + fraction*(v_hi - v_lo);
	if (sum_flag) v[i][v_dim] += vramp;
	else v[i][v_dim] = vramp;
	}
	}

	/* ----------------------------------------------------------------------
	zero linear or angular momentum of a group
	if using rigid/small requires init of entire system since
	its methods perform forward/reverse comm,
	comm::init needs neighbor::init needs pair::init needs kspace::init, etc
	also requires setup_pre_neighbor call to setup bodies
	------------------------------------------------------------------------- */

	void Velocity::zero(int narg, char **arg)
	{
	if (strcmp(arg[0],"linear") == 0) {
	if (rfix < 0) zero_momentum();
	else {
	if (strcmp(modify->fix[rfix]->style,"rigid/small") == 0) {
	lmp->init();
	modify->fix[rfix]->setup_pre_neighbor();
	modify->fix[rfix]->zero_momentum();
	} else if (strstr(modify->fix[rfix]->style,"rigid")) {
	modify->fix[rfix]->zero_momentum();
	} else error->all(FLERR,"Velocity rigid used with non-rigid fix-ID");
	}

	} else if (strcmp(arg[0],"angular") == 0) {
	if (rfix < 0) zero_rotation();
	else {
	if (strcmp(modify->fix[rfix]->style,"rigid/small") == 0) {
	lmp->init();
	modify->fix[rfix]->setup_pre_neighbor();
	modify->fix[rfix]->zero_rotation();
	} else if (strstr(modify->fix[rfix]->style,"rigid")) {
	modify->fix[rfix]->zero_rotation();
	} else error->all(FLERR,"Velocity rigid used with non-rigid fix-ID");
	}

	} else error->all(FLERR,"Illegal velocity command");
	}

	/* ----------------------------------------------------------------------
	rescale velocities of group atoms to t_new from t_old
	------------------------------------------------------------------------- */

	void Velocity::rescale(double t_old, double t_new)
	{
	if (t_old == 0.0) error->all(FLERR,"Attempting to rescale a 0.0 temperature");

	double factor = sqrt(t_new/t_old);

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

	for (int i = 0; i < nlocal; i++)
	if (mask[i] & groupbit) {
	v[i][0] *= factor;
	v[i][1] *= factor;
	v[i][2] *= factor;
	}
	}

	/* ----------------------------------------------------------------------
	zero the linear momentum of a group of atoms by adjusting v by -Vcm
	------------------------------------------------------------------------- */

	void Velocity::zero_momentum()
	{
	// cannot have no atoms in group

	if (group->count(igroup) == 0)
	error->all(FLERR,"Cannot zero momentum of no atoms");

	// compute velocity of center-of-mass of group

	double masstotal = group->mass(igroup);
	double vcm[3];
	group->vcm(igroup,masstotal,vcm);

	// adjust velocities by vcm to zero linear momentum

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

	for (int i = 0; i < nlocal; i++)
	if (mask[i] & groupbit) {
	v[i][0] -= vcm[0];
	v[i][1] -= vcm[1];
	v[i][2] -= vcm[2];
	}
	}

	/* ----------------------------------------------------------------------
	zero the angular momentum of a group of atoms by adjusting v by -(w x r)
	------------------------------------------------------------------------- */

	void Velocity::zero_rotation()
	{
	int i;

	// cannot have no atoms in group

	if (group->count(igroup) == 0)
	error->all(FLERR,"Cannot zero momentum of no atoms");

	// compute omega (angular velocity) of group around center-of-mass

	double xcm[3],angmom[3],inertia[3][3],omega[3];
	double masstotal = group->mass(igroup);
	group->xcm(igroup,masstotal,xcm);
	group->angmom(igroup,xcm,angmom);
	group->inertia(igroup,xcm,inertia);
	group->omega(angmom,inertia,omega);

	// adjust velocities to zero omega
	// vnew_i = v_i - w x r_i
	// must use unwrapped coords to compute r_i correctly

	double **x = atom->x;
	double **v = atom->v;
	int *mask = atom->mask;
	imageint *image = atom->image;
	int nlocal = atom->nlocal;

	double dx,dy,dz;
	double unwrap[3];

	for (i = 0; i < nlocal; i++)
	if (mask[i] & groupbit) {
	domain->unmap(x[i],image[i],unwrap);
	dx = unwrap[0] - xcm[0];
	dy = unwrap[1] - xcm[1];
	dz = unwrap[2] - xcm[2];
	v[i][0] -= omega[1]dz - omega[2]dy;
	v[i][1] -= omega[2]dx - omega[0]dz;
	v[i][2] -= omega[0]dy - omega[1]dx;
	}
	}

	/* ----------------------------------------------------------------------
	parse optional parameters at end of velocity input line
	------------------------------------------------------------------------- */

	void Velocity::options(int narg, char **arg)
	{
	if (narg < 0) error->all(FLERR,"Illegal velocity command");

	int iarg = 0;
	while (iarg < narg) {
	if (strcmp(arg[iarg],"dist") == 0) {
	if (iarg+2 > narg) error->all(FLERR,"Illegal velocity command");
	if (strcmp(arg[iarg+1],"uniform") == 0) dist_flag = 0;
	else if (strcmp(arg[iarg+1],"gaussian") == 0) dist_flag = 1;
	else error->all(FLERR,"Illegal velocity command");
	iarg += 2;
	} else if (strcmp(arg[iarg],"sum") == 0) {
	if (iarg+2 > narg) error->all(FLERR,"Illegal velocity command");
	if (strcmp(arg[iarg+1],"no") == 0) sum_flag = 0;
	else if (strcmp(arg[iarg+1],"yes") == 0) sum_flag = 1;
	else error->all(FLERR,"Illegal velocity command");
	iarg += 2;
	} else if (strcmp(arg[iarg],"mom") == 0) {
	if (iarg+2 > narg) error->all(FLERR,"Illegal velocity command");
	if (strcmp(arg[iarg+1],"no") == 0) momentum_flag = 0;
	else if (strcmp(arg[iarg+1],"yes") == 0) momentum_flag = 1;
	else error->all(FLERR,"Illegal velocity command");
	iarg += 2;
	} else if (strcmp(arg[iarg],"rot") == 0) {
	if (iarg+2 > narg) error->all(FLERR,"Illegal velocity command");
	if (strcmp(arg[iarg+1],"no") == 0) rotation_flag = 0;
	else if (strcmp(arg[iarg+1],"yes") == 0) rotation_flag = 1;
	else error->all(FLERR,"Illegal velocity command");
	iarg += 2;
	} else if (strcmp(arg[iarg],"temp") == 0) {
	if (iarg+2 > narg) error->all(FLERR,"Illegal velocity command");
	int icompute;
	for (icompute = 0; icompute < modify->ncompute; icompute++)
	if (strcmp(arg[iarg+1],modify->compute[icompute]->id) == 0) break;
	if (icompute == modify->ncompute)
	error->all(FLERR,"Could not find velocity temperature ID");
	temperature = modify->compute[icompute];
	if (temperature->tempflag == 0)
	error->all(FLERR,
	"Velocity temperature ID does not compute temperature");
	iarg += 2;
	} else if (strcmp(arg[iarg],"loop") == 0) {
	if (iarg+2 > narg) error->all(FLERR,"Illegal velocity command");
	if (strcmp(arg[iarg+1],"all") == 0) loop_flag = ALL;
	else if (strcmp(arg[iarg+1],"local") == 0) loop_flag = LOCAL;
	else if (strcmp(arg[iarg+1],"geom") == 0) loop_flag = GEOM;
	else error->all(FLERR,"Illegal velocity command");
	iarg += 2;
	} else if (strcmp(arg[iarg],"rigid") == 0) {
	if (iarg+2 > narg) error->all(FLERR,"Illegal velocity command");
	rfix = modify->find_fix(arg[iarg+1]);
	if (rfix < 0) error->all(FLERR,"Fix ID for velocity does not exist");
	iarg += 2;
	} else if (strcmp(arg[iarg],"units") == 0) {
	if (iarg+2 > narg) error->all(FLERR,"Illegal velocity command");
	if (strcmp(arg[iarg+1],"box") == 0) scale_flag = 0;
	else if (strcmp(arg[iarg+1],"lattice") == 0) scale_flag = 1;
	else error->all(FLERR,"Illegal velocity command");
	iarg += 2;
	} else error->all(FLERR,"Illegal velocity command");
	}
	}

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