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fix_momentum.cpp
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Sat, Jun 28, 08:38

fix_momentum.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 "stdlib.h"
#include "string.h"
#include "fix_momentum.h"
#include "atom.h"
#include "domain.h"
#include "group.h"
#include "error.h"
using namespace LAMMPS_NS;
/* ----------------------------------------------------------------------
Contributing author: Naveen Michaud-Agrawal (Johns Hopkins U)
------------------------------------------------------------------------- */
/* ---------------------------------------------------------------------- */
FixMomentum::FixMomentum(LAMMPS *lmp, int narg, char **arg) :
Fix(lmp, narg, arg)
{
if (narg < 4) error->all("Illegal fix momentum command");
nevery = atoi(arg[3]);
if (nevery <= 0) error->all("Illegal fix momentum command");
linear = angular = 0;
int iarg = 4;
while (iarg < narg) {
if (strcmp(arg[iarg],"linear") == 0) {
if (iarg+4 > narg) error->all("Illegal fix momentum command");
linear = 1;
xflag = atoi(arg[iarg+1]);
yflag = atoi(arg[iarg+2]);
zflag = atoi(arg[iarg+3]);
iarg += 4;
} else if (strcmp(arg[iarg],"angular") == 0) {
angular = 1;
iarg += 1;
} else error->all("Illegal fix momentum command");
}
if (linear == 0 && angular == 0)
error->all("Illegal fix momentum command");
if (linear)
if (xflag < 0 || xflag > 1 || yflag < 0 || yflag > 1 ||
zflag < 0 || zflag > 1) error->all("Illegal fix momentum command");
// cannot have 0 atoms in group
if (group->count(igroup) == 0.0)
error->all("Fix momentum group has no atoms");
}
/* ---------------------------------------------------------------------- */
int FixMomentum::setmask()
{
int mask = 0;
mask |= END_OF_STEP;
return mask;
}
/* ---------------------------------------------------------------------- */
void FixMomentum::init()
{
masstotal = group->mass(igroup);
}
/* ---------------------------------------------------------------------- */
void FixMomentum::end_of_step()
{
if (linear) {
double vcm[3];
group->vcm(igroup,masstotal,vcm);
// adjust velocities by vcm to zero linear momentum
// only adjust a component if flag is set
double **v = atom->v;
int *mask = atom->mask;
int nlocal = atom->nlocal;
for (int i = 0; i < nlocal; i++)
if (mask[i] & groupbit) {
if (xflag) v[i][0] -= vcm[0];
if (yflag) v[i][1] -= vcm[1];
if (zflag) v[i][2] -= vcm[2];
}
}
if (angular) {
double xcm[3],angmom[3],inertia[3][3],omega[3];
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;
int *image = atom->image;
int nlocal = atom->nlocal;
int xbox,ybox,zbox;
double dx,dy,dz;
double xprd = domain->xprd;
double yprd = domain->yprd;
double zprd = domain->zprd;
for (int i = 0; i < nlocal; i++)
if (mask[i] & groupbit) {
xbox = (image[i] & 1023) - 512;
ybox = (image[i] >> 10 & 1023) - 512;
zbox = (image[i] >> 20) - 512;
dx = (x[i][0] + xbox*xprd) - xcm[0];
dy = (x[i][1] + ybox*yprd) - xcm[1];
dz = (x[i][2] + zbox*zprd) - 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;
}
}
}

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