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fix_rhoKUmbrella.cpp
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Tue, Nov 26, 08:04

fix_rhoKUmbrella.cpp
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/*
fix_rhoK_umbrella.cpp
A fix to do umbrella sampling on rho(k).
The usage is as follows:
fix [name] [groupID] rhoKUmbrella [nx] [ny] [nz] [kappa = spring constant] [rhoK0]
where k_i = (2 pi / L_i) * n_i
Written by Ulf Pedersen and Patrick Varilly, 4 Feb 2010
Tweaked for LAMMPS 15 Jan 2010 version by Ulf Pedersen, 19 Aug 2010
Tweaked again March 4th 2012 by Ulf Pedersen.
*/
#include "fix_rhoKUmbrella.h"
#include "error.h"
#include "update.h"
#include "respa.h"
#include "atom.h"
#include "domain.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <math.h>
using namespace LAMMPS_NS;
using namespace FixConst;
// Constructor: all the parameters to this fix specified in
// the LAMMPS input get passed in
FixRhoKUmbrella::FixRhoKUmbrella( LAMMPS* inLMP, int inArgc, char** inArgv )
: Fix( inLMP, inArgc, inArgv )
{
// Check arguments
if( inArgc != 8 )
error->all(FLERR,"Illegal fix rhoKUmbrella command" );
// Set up fix flags
scalar_flag = 1; // have compute_scalar
vector_flag = 1; // have compute_vector...
size_vector = 3; // ...with this many components
global_freq = 1; // whose value can be computed at every timestep
//scalar_vector_freq = 1; // OLD lammps: whose value can be computed at every timestep
thermo_energy = 1; // this fix changes system's potential energy
extscalar = 0; // but the deltaPE might not scale with # of atoms
extvector = 0; // neither do the components of the vector
// Parse fix options
int n[3];
n[0] = atoi( inArgv[3] );
n[1] = atoi( inArgv[4] );
n[2] = atoi( inArgv[5] );
mK[0] = n[0]*(2*M_PI / (domain->boxhi[0] - domain->boxlo[0]));
mK[1] = n[1]*(2*M_PI / (domain->boxhi[1] - domain->boxlo[1]));
mK[2] = n[2]*(2*M_PI / (domain->boxhi[2] - domain->boxlo[2]));
mKappa = atof( inArgv[6] );
mRhoK0 = atof( inArgv[7] );
}
FixRhoKUmbrella::~FixRhoKUmbrella()
{
}
// Methods that this fix implements
// --------------------------------
// Tells LAMMPS where this fix should act
int
FixRhoKUmbrella::setmask()
{
int mask = 0;
// This fix modifies forces...
mask |= POST_FORCE;
mask |= POST_FORCE_RESPA;
mask |= MIN_POST_FORCE;
// ...and potential energies
mask |= THERMO_ENERGY;
return mask;
}
/*int FixRhoKUmbrella::setmask()
{
int mask = 0;
mask |= POST_FORCE;
mask |= POST_FORCE_RESPA;
mask |= MIN_POST_FORCE;
return mask;
}*/
// Initializes the fix at the beginning of a run
void
FixRhoKUmbrella::init()
{
// RESPA boilerplate
if( strcmp( update->integrate_style, "respa" ) == 0 )
mNLevelsRESPA = ((Respa *) update->integrate)->nlevels;
// Count the number of affected particles
int nThisLocal = 0;
int *mask = atom->mask;
int nlocal = atom->nlocal;
for( int i = 0; i < nlocal; i++ ) { // Iterate through all atoms on this CPU
if( mask[i] & groupbit ) { // ...only those affected by this fix
nThisLocal++;
}
}
MPI_Allreduce( &nThisLocal, &mNThis,
1, MPI_INT, MPI_SUM, world );
mSqrtNThis = sqrt( mNThis );
}
// Initial application of the fix to a system (when doing MD)
void
FixRhoKUmbrella::setup( int inVFlag )
{
if( strcmp( update->integrate_style, "verlet" ) == 0 )
post_force( inVFlag );
else
{
((Respa *) update->integrate)->copy_flevel_f( mNLevelsRESPA - 1 );
post_force_respa( inVFlag, mNLevelsRESPA - 1,0 );
((Respa *) update->integrate)->copy_f_flevel( mNLevelsRESPA - 1 );
}
}
// Initial application of the fix to a system (when doing minimization)
void
FixRhoKUmbrella::min_setup( int inVFlag )
{
post_force( inVFlag );
}
// Modify the forces calculated in the main force loop of ordinary MD
void
FixRhoKUmbrella::post_force( int inVFlag )
{
double **x = atom->x;
double **f = atom->f;
int *mask = atom->mask;
int nlocal = atom->nlocal;
// Loop over locally-owned atoms affected by this fix and calculate the
// partial rhoK's
mRhoKLocal[0] = 0.0;
mRhoKLocal[1] = 0.0;
for( int i = 0; i < nlocal; i++ ) { // Iterate through all atoms on this CPU
if( mask[i] & groupbit ) { // ...only those affected by this fix
// rho_k = sum_i exp( - i k.r_i )
mRhoKLocal[0] += cos( mK[0]*x[i][0] + mK[1]*x[i][1] + mK[2]*x[i][2] );
mRhoKLocal[1] -= sin( mK[0]*x[i][0] + mK[1]*x[i][1] + mK[2]*x[i][2] );
}
}
// Now calculate mRhoKGlobal
MPI_Allreduce( mRhoKLocal, mRhoKGlobal,
2, MPI_DOUBLE, MPI_SUM, world );
// WARNING!!!!! < \sum_{i,j} e^{-ik.(r_i - r_j)} > ~ N, so
// we define rho_k as (1 / sqrt(N)) \sum_i e^{-i k.r_i}, so that
// <rho_k^2> is intensive.
//
// Don't forget this two years from now when you change the system size!!!
mRhoKGlobal[0] /= mSqrtNThis;
mRhoKGlobal[1] /= mSqrtNThis;
// We'll need magnitude of rho_k
double rhoK = sqrt( mRhoKGlobal[0]*mRhoKGlobal[0]
+ mRhoKGlobal[1]*mRhoKGlobal[1] );
for( int i = 0; i < nlocal; i++ ) { // Iterate through all atoms on this CPU
if( mask[i] & groupbit ) { // ...only those affected by this fix
// Calculate forces
// U = kappa/2 ( |rho_k| - rho_k^0 )^2
// f_i = -grad_i U = -kappa ( |rho_k| - rho_k^0 ) grad_i |rho_k|
// grad_i |rho_k| = Re( rho_k* (-i k e^{-i k . r_i} / sqrt(N)) ) / |rho_k|
//
// In terms of real and imag parts of rho_k,
//
// Re( rho_k* (-i k e^{-i k . r_i}) ) =
// (- Re[rho_k] * sin( k . r_i ) - Im[rho_k] * cos( k . r_i )) * k
double sinKRi = sin( mK[0]*x[i][0] + mK[1]*x[i][1] + mK[2]*x[i][2] );
double cosKRi = cos( mK[0]*x[i][0] + mK[1]*x[i][1] + mK[2]*x[i][2] );
double prefactor = mKappa * ( rhoK - mRhoK0 ) / rhoK
* (-mRhoKGlobal[0]*sinKRi - mRhoKGlobal[1]*cosKRi) / mSqrtNThis;
f[i][0] -= prefactor * mK[0];
f[i][1] -= prefactor * mK[1];
f[i][2] -= prefactor * mK[2];
}
}
}
// Forces in RESPA loop
void
FixRhoKUmbrella::post_force_respa( int inVFlag, int inILevel, int inILoop )
{
if( inILevel == mNLevelsRESPA - 1 )
post_force( inVFlag );
}
// Forces in minimization loop
void
FixRhoKUmbrella::min_post_force( int inVFlag )
{
post_force( inVFlag );
}
// Compute the change in the potential energy induced by this fix
double
FixRhoKUmbrella::compute_scalar()
{
double rhoK = sqrt( mRhoKGlobal[0]*mRhoKGlobal[0]
+ mRhoKGlobal[1]*mRhoKGlobal[1] );
return 0.5 * mKappa * (rhoK - mRhoK0) * (rhoK - mRhoK0);
}
// Compute the ith component of the vector
double
FixRhoKUmbrella::compute_vector( int inI )
{
if( inI == 0 )
return mRhoKGlobal[0]; // Real part
else if( inI == 1 )
return mRhoKGlobal[1]; // Imagniary part
else if( inI == 2 )
return sqrt( mRhoKGlobal[0]*mRhoKGlobal[0]
+ mRhoKGlobal[1]*mRhoKGlobal[1] );
else
return 12345.0;
}

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