ReferenceVariableStochasticDynamics.cpp

/* Portions copyright (c) 2006 Stanford University and Simbios.
 * Contributors: Pande Group
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the
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 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject
 * to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included
 * in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
 * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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#include <cstring>
#include <sstream>

#include "../SimTKUtilities/SimTKOpenMMCommon.h"
#include "../SimTKUtilities/SimTKOpenMMLog.h"
#include "../SimTKUtilities/SimTKOpenMMUtilities.h"
#include "ReferenceVariableStochasticDynamics.h"

#include <cstdio>

using std::vector;
using OpenMM::RealVec;

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

   ReferenceVariableStochasticDynamics constructor

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

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

ReferenceVariableStochasticDynamics::ReferenceVariableStochasticDynamics( int numberOfAtoms,
                                                          RealOpenMM tau, RealOpenMM temperature,
                                                          RealOpenMM accuracy ) :
           ReferenceDynamics(numberOfAtoms, 0.0f, temperature), _tau(tau), _accuracy(accuracy) {

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

   static const char* methodName      = "\nReferenceVariableStochasticDynamics::ReferenceVariableStochasticDynamics";

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

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

   // ensure tau is not zero -- if it is print warning message

   if( _tau == zero ){

      std::stringstream message;
      message << methodName;
      message << " input tau value=" << tau << " is invalid -- setting to 1.";
      SimTKOpenMMLog::printError( message );

      _tau = one;

   }
   xPrime.resize(numberOfAtoms);
   inverseMasses.resize(numberOfAtoms);
}

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

   ReferenceVariableStochasticDynamics destructor

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

ReferenceVariableStochasticDynamics::~ReferenceVariableStochasticDynamics( ){

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

   // static const char* methodName = "\nReferenceVariableStochasticDynamics::~ReferenceVariableStochasticDynamics";

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

}

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

   Get the required accuracy

   @return accuracy

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

RealOpenMM ReferenceVariableStochasticDynamics::getAccuracy( void ) const {
    return _accuracy;
}

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

   Set the required accuracy

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

void ReferenceVariableStochasticDynamics::setAccuracy( RealOpenMM accuracy ) {
    _accuracy = accuracy;
}

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

   Get tau

   @return tau

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

RealOpenMM ReferenceVariableStochasticDynamics::getTau( void ) const {

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

   // static const char* methodName  = "\nReferenceVariableStochasticDynamics::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 masses              atom masses
   @param inverseMasses       inverse atom masses
   @param xPrime              xPrime
   @param maxStepSize         maximum time step

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

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

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

   static const char* methodName  = "\nReferenceVariableStochasticDynamics::updatePart1";

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


   // first-time-through initialization

   if( getTimeStep() == 0 ){

      std::stringstream message;
      message << methodName;
      int errors = 0;

      // invert masses

      for( int ii = 0; ii < numberOfAtoms; ii++ ){
         if( masses[ii] <= 0 ){
            message << "mass at atom index=" << ii << " (" << masses[ii] << ") is <= 0" << std::endl;
            errors++;
         } else {
            inverseMasses[ii] = 1/masses[ii];
         }
      }

      // exit if errors

      if( errors ){
         SimTKOpenMMLog::printError( message );
      }
   }

   // Select the step size to use
    RealOpenMM error = 0;
    for (int i = 0; i < numberOfAtoms; ++i) {
        for (int j = 0; j < 3; ++j) {
            RealOpenMM xerror = inverseMasses[i]*forces[i][j];
            error += xerror*xerror;
        }
    }
    error = SQRT(error/(numberOfAtoms*3));
    RealOpenMM newStepSize = SQRT(getAccuracy()/error);
    if (getDeltaT() > 0.0f)
        newStepSize = std::min(newStepSize, getDeltaT()*2.0f); // For safety, limit how quickly dt can increase.
    if (newStepSize > getDeltaT() && newStepSize < 1.2f*getDeltaT())
        newStepSize = getDeltaT(); // Keeping dt constant between steps improves the behavior of the integrator.
    if (newStepSize > maxStepSize)
        newStepSize = maxStepSize;
    setDeltaT(newStepSize);
 
    // 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++) {
      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 ReferenceVariableStochasticDynamics::updatePart2( int numberOfAtoms, vector<RealVec>& atomCoordinates,
                                              vector<RealVec>& velocities,
                                              vector<RealVec>& forces, vector<RealOpenMM>& inverseMasses,
                                              vector<RealVec>& xPrime ){

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

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

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

   // perform second update

   for (int ii = 0; ii < numberOfAtoms; ii++) {
      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 numberOfAtoms       number of atoms
   @param atomCoordinates     atom coordinates
   @param velocities          velocities
   @param forces              forces
   @param masses              atom masses

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

void ReferenceVariableStochasticDynamics::update( int numberOfAtoms, vector<RealVec>& atomCoordinates,
                                          vector<RealVec>& velocities,
                                          vector<RealVec>& forces, vector<RealOpenMM>& masses, RealOpenMM maxStepSize ){

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

   //static const char* methodName      = "\nReferenceVariableStochasticDynamics::update";

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

   // 1st update

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

   // 2nd update

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

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

   // copy xPrime -> atomCoordinates

   for( int ii = 0; ii < numberOfAtoms; ii++ ){
      atomCoordinates[ii][0] = xPrime[ii][0];
      atomCoordinates[ii][1] = xPrime[ii][1];
      atomCoordinates[ii][2] = xPrime[ii][2];
   }

   incrementTimeStep();

}