compute_gyration.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 "compute_gyration.h"
	#include "update.h"
	#include "group.h"
	#include "error.h"

	using namespace LAMMPS_NS;

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

	ComputeGyration::ComputeGyration(LAMMPS lmp, int narg, char *arg) :
	Compute(lmp, narg, arg)
	{
	if (narg != 3) error->all(FLERR,"Illegal compute gyration command");

	scalar_flag = 1;
	extscalar = 0;
	}

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

	void ComputeGyration::init()
	{
	masstotal = group->mass(igroup);
	}

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

	double ComputeGyration::compute_scalar()
	{
	invoked_scalar = update->ntimestep;

	double xcm[3];
	group->xcm(igroup,masstotal,xcm);
	scalar = group->gyration(igroup,masstotal,xcm);
	return scalar;
	}