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compute_msd.cpp
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Sat, Aug 31, 11:25

compute_msd.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 "string.h"
#include "compute_msd.h"
#include "atom.h"
#include "update.h"
#include "group.h"
#include "domain.h"
#include "modify.h"
#include "fix.h"
#include "error.h"
using namespace LAMMPS_NS;
/* ---------------------------------------------------------------------- */
ComputeMSD::ComputeMSD(LAMMPS *lmp, int narg, char **arg) :
Compute(lmp, narg, arg)
{
if (narg < 3) error->all("Illegal compute msd command");
vector_flag = 1;
size_vector = 4;
extvector = 0;
// optional args
comflag = 0;
int iarg = 3;
while (iarg < narg) {
if (strcmp(arg[iarg],"com") == 0) {
if (iarg+2 > narg) error->all("Illegal compute msd command");
if (strcmp(arg[iarg+1],"no") == 0) comflag = 0;
else if (strcmp(arg[iarg+1],"yes") == 0) comflag = 1;
else error->all("Illegal compute msd command");
iarg += 2;
} else error->all("Illegal compute msd command");
}
// create a new fix store/coord style with or without com keyword
// id = compute-ID + store_coord, fix group = compute group
int n = strlen(id) + strlen("_store_coord") + 1;
id_fix = new char[n];
strcpy(id_fix,id);
strcat(id_fix,"_store_coord");
char **newarg = new char*[5];
newarg[0] = id_fix;
newarg[1] = group->names[igroup];
newarg[2] = (char *) "store/coord";
newarg[3] = (char *) "com";
newarg[4] = (char *) "yes";
if (comflag) modify->add_fix(5,newarg);
else modify->add_fix(3,newarg);
delete [] newarg;
vector = new double[4];
}
/* ---------------------------------------------------------------------- */
ComputeMSD::~ComputeMSD()
{
// check nfix in case all fixes have already been deleted
if (modify->nfix) modify->delete_fix(id_fix);
delete [] id_fix;
delete [] vector;
}
/* ---------------------------------------------------------------------- */
void ComputeMSD::init()
{
// set fix which stores original atom coords
int ifix = modify->find_fix(id_fix);
if (ifix < 0) error->all("Could not find compute msd fix ID");
fix = modify->fix[ifix];
// nmsd = # of atoms in group
int *mask = atom->mask;
int nlocal = atom->nlocal;
nmsd = 0;
for (int i = 0; i < nlocal; i++)
if (mask[i] & groupbit) nmsd++;
int nmsd_all;
MPI_Allreduce(&nmsd,&nmsd_all,1,MPI_INT,MPI_SUM,world);
nmsd = nmsd_all;
masstotal = group->mass(igroup);
}
/* ---------------------------------------------------------------------- */
void ComputeMSD::compute_vector()
{
invoked_vector = update->ntimestep;
// cm = current center of mass
double cm[3];
if (comflag) group->xcm(igroup,masstotal,cm);
else cm[0] = cm[1] = cm[2] = 0.0;
// dx,dy,dz = displacement of atom from original position
// original unwrapped position is stored by fix
// relative to center of mass if comflag is set
// for triclinic, need to unwrap current atom coord via h matrix
double **xoriginal = fix->array_atom;
double **x = atom->x;
int *mask = atom->mask;
int *image = atom->image;
int nlocal = atom->nlocal;
double *h = domain->h;
double xprd = domain->xprd;
double yprd = domain->yprd;
double zprd = domain->zprd;
double dx,dy,dz;
int xbox,ybox,zbox;
double msd[4];
msd[0] = msd[1] = msd[2] = msd[3] = 0.0;
if (domain->triclinic == 0) {
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 - cm[0] - xoriginal[i][0];
dy = x[i][1] + ybox*yprd - cm[1] - xoriginal[i][1];
dz = x[i][2] + zbox*zprd - cm[2] - xoriginal[i][2];
msd[0] += dx*dx;
msd[1] += dy*dy;
msd[2] += dz*dz;
msd[3] += dx*dx + dy*dy + dz*dz;
}
} else {
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] + h[0]*xbox + h[5]*ybox + h[4]*zbox -
cm[0] - xoriginal[i][0];
dy = x[i][1] + h[1]*ybox + h[3]*zbox - cm[1] - xoriginal[i][1];
dz = x[i][2] + h[2]*zbox - cm[2] - xoriginal[i][2];
msd[0] += dx*dx;
msd[1] += dy*dy;
msd[2] += dz*dz;
msd[3] += dx*dx + dy*dy + dz*dz;
}
}
MPI_Allreduce(msd,vector,4,MPI_DOUBLE,MPI_SUM,world);
if (nmsd) {
vector[0] /= nmsd;
vector[1] /= nmsd;
vector[2] /= nmsd;
vector[3] /= nmsd;
}
}

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