ReferenceStochasticDynamics.cpp

/* Portions copyright (c) 2006-2013 Stanford University and Simbios.
 * Contributors: Pande Group
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#include <cstring>
#include <sstream>

#include "SimTKOpenMMCommon.h"
#include "SimTKOpenMMUtilities.h"
#include "ReferenceStochasticDynamics.h"
#include "ReferenceVirtualSites.h"
#include "openmm/OpenMMException.h"

#include <cstdio>

using std::vector;
using namespace OpenMM;

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

   ReferenceStochasticDynamics constructor

   @param numberOfAtoms  number of atoms
   @param deltaT         delta t for dynamics
   @param tau            viscosity(?)
   @param temperature    temperature

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

ReferenceStochasticDynamics::ReferenceStochasticDynamics( int numberOfAtoms,
                                                          RealOpenMM deltaT, RealOpenMM tau,
                                                          RealOpenMM temperature ) : 
           ReferenceDynamics( numberOfAtoms, deltaT, temperature ), _tau( tau ) {
   if (tau <= 0) {
      std::stringstream message;
      message << "illegal tau value: " << tau;
      throw OpenMMException(message.str());
   }
   xPrime.resize(numberOfAtoms);
   inverseMasses.resize(numberOfAtoms);
}

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

   ReferenceStochasticDynamics destructor

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

ReferenceStochasticDynamics::~ReferenceStochasticDynamics( ){

   // ---------------------------------------------------------------------------------------

   // static const char* methodName = "\nReferenceStochasticDynamics::~ReferenceStochasticDynamics";

   // ---------------------------------------------------------------------------------------

}

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

   Get tau

   @return tau

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

RealOpenMM ReferenceStochasticDynamics::getTau( void ) const {

   // ---------------------------------------------------------------------------------------

   // static const char* methodName  = "\nReferenceStochasticDynamics::getTau";

   // ---------------------------------------------------------------------------------------

   return _tau;
}

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

   First SD update; based on code in update.c do_update_sd() Gromacs 3.1.4

   @param numberOfAtoms       number of atoms
   @param atomCoordinates     atom coordinates
   @param velocities          velocities
   @param forces              forces
   @param inverseMasses       inverse atom masses
   @param xPrime              xPrime

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

void ReferenceStochasticDynamics::updatePart1( int numberOfAtoms, vector<RealVec>& atomCoordinates,
                                              vector<RealVec>& velocities,
                                              vector<RealVec>& forces, vector<RealOpenMM>& inverseMasses,
                                              vector<RealVec>& xPrime ){

   // ---------------------------------------------------------------------------------------

   //static const char* methodName  = "\nReferenceStochasticDynamics::updatePart1";

   // ---------------------------------------------------------------------------------------

   // perform first update

   RealOpenMM tau = getTau();
   const RealOpenMM vscale = EXP(-getDeltaT()/tau);
   const RealOpenMM fscale = (1-vscale)*tau;
   const RealOpenMM kT = BOLTZ*getTemperature();
   const RealOpenMM noisescale = SQRT(2*kT/tau)*SQRT(0.5*(1-vscale*vscale)*tau);

   for (int ii = 0; ii < numberOfAtoms; ii++) {
       if (inverseMasses[ii] != 0.0) {
           RealOpenMM sqrtInvMass = SQRT(inverseMasses[ii]);
           for (int jj = 0; jj < 3; jj++) {
               velocities[ii][jj]  = vscale*velocities[ii][jj] + fscale*inverseMasses[ii]*forces[ii][jj] + noisescale*sqrtInvMass*SimTKOpenMMUtilities::getNormallyDistributedRandomNumber();
           }
       }
   }
}

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

   Second update; based on code in update.c do_update_sd() w/ bFirstHalf = false in Gromacs 3.1.4

   @param numberOfAtoms       number of atoms
   @param atomCoordinates     atom coordinates
   @param velocities          velocities
   @param forces              forces
   @param masses              atom masses

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

void ReferenceStochasticDynamics::updatePart2( int numberOfAtoms, vector<RealVec>& atomCoordinates,
                                              vector<RealVec>& velocities,
                                              vector<RealVec>& forces, vector<RealOpenMM>& inverseMasses,
                                              vector<RealVec>& xPrime ){

   // ---------------------------------------------------------------------------------------

   //static const char* methodName  = "\nReferenceStochasticDynamics::updatePart2";

   // ---------------------------------------------------------------------------------------

   // perform second update

   for (int ii = 0; ii < numberOfAtoms; ii++) {
       if (inverseMasses[ii] != 0.0)
           for (int jj = 0; jj < 3; jj++)
               xPrime[ii][jj] = atomCoordinates[ii][jj]+getDeltaT()*velocities[ii][jj];
   }
}

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

   Update -- driver routine for performing stochastic dynamics update of coordinates
   and velocities

   @param system              the System to be integrated
   @param atomCoordinates     atom coordinates
   @param velocities          velocities
   @param forces              forces
   @param masses              atom masses

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

void ReferenceStochasticDynamics::update(const OpenMM::System& system, vector<RealVec>& atomCoordinates,
                                          vector<RealVec>& velocities, vector<RealVec>& forces, vector<RealOpenMM>& masses, RealOpenMM tolerance) {

   // ---------------------------------------------------------------------------------------

   static const char* methodName      = "\nReferenceStochasticDynamics::update";

   static const RealOpenMM zero       =  0.0;
   static const RealOpenMM one        =  1.0;

   // ---------------------------------------------------------------------------------------

   // first-time-through initialization

   int numberOfAtoms = system.getNumParticles();
   if( getTimeStep() == 0 ){
      // invert masses

      for( int ii = 0; ii < numberOfAtoms; ii++ ){
         if (masses[ii] == zero)
             inverseMasses[ii] = zero;
         else
             inverseMasses[ii] = one/masses[ii];
      }
   }

   // 1st update

   updatePart1( numberOfAtoms, atomCoordinates, velocities, forces, inverseMasses, xPrime );

   // 2nd update

   updatePart2( numberOfAtoms, atomCoordinates, velocities, forces, inverseMasses, xPrime );

   ReferenceConstraintAlgorithm* referenceConstraintAlgorithm = getReferenceConstraintAlgorithm();
   if (referenceConstraintAlgorithm)
      referenceConstraintAlgorithm->apply(atomCoordinates, xPrime, inverseMasses, tolerance);

   // copy xPrime -> atomCoordinates

   RealOpenMM invStepSize = 1.0/getDeltaT();
   for (int i = 0; i < numberOfAtoms; ++i)
       if (masses[i] != zero)
           for (int j = 0; j < 3; ++j) {
               velocities[i][j] = invStepSize*(xPrime[i][j]-atomCoordinates[i][j]);
               atomCoordinates[i][j] = xPrime[i][j];
           }

   ReferenceVirtualSites::computePositions(system, atomCoordinates);
   incrementTimeStep();
}