ReferenceVariableVerletDynamics.cpp

/* Portions copyright (c) 2006-2013 Stanford University and Simbios.
 * Contributors: Peter Eastman, Pande Group
 *
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 * permit persons to whom the Software is furnished to do so, subject
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 *
 * The above copyright notice and this permission notice shall be included
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 *
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 * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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#include <string.h>
#include <sstream>
#include <algorithm>

#include "SimTKOpenMMCommon.h"
#include "SimTKOpenMMLog.h"
#include "SimTKOpenMMUtilities.h"
#include "ReferenceVariableVerletDynamics.h"
#include "ReferenceVirtualSites.h"

using std::vector;
using namespace OpenMM;

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

   ReferenceVariableVerletDynamics constructor

   @param numberOfAtoms  number of atoms
   @param deltaT         initial delta t for dynamics
   @param accuracy       required accuracy

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

ReferenceVariableVerletDynamics::ReferenceVariableVerletDynamics( int numberOfAtoms, RealOpenMM accuracy ) :
           ReferenceDynamics( numberOfAtoms, 0.0f, 0.0f ), _accuracy(accuracy) {
    xPrime.resize(numberOfAtoms);
    inverseMasses.resize(numberOfAtoms);
}

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

   ReferenceVariableVerletDynamics destructor

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

ReferenceVariableVerletDynamics::~ReferenceVariableVerletDynamics( ){

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

   // static const char* methodName = "\nReferenceVariableVerletDynamics::~ReferenceVariableVerletDynamics";

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

}

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

   Get the required accuracy

   @return accuracy

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

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

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

   Set the required accuracy

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

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

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

   Update -- driver routine for performing Verlet 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
   @param maxStepSize         maximum time step

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

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

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

    static const char* methodName      = "\nReferenceVariableVerletDynamics::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];
       }
    }

    RealOpenMM error = zero;
    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;
    RealOpenMM vstep = 0.5f*(newStepSize+getDeltaT()); // The time interval by which to advance the velocities
    setDeltaT(newStepSize);
    for (int i = 0; i < numberOfAtoms; ++i) {
        if (masses[i] != zero)
            for (int j = 0; j < 3; ++j) {
                RealOpenMM vPrime = velocities[i][j] + inverseMasses[i]*forces[i][j]*vstep;
                xPrime[i][j] = atomCoordinates[i][j] + vPrime*getDeltaT();
            }
    }
    ReferenceConstraintAlgorithm* referenceConstraintAlgorithm = getReferenceConstraintAlgorithm();
    if (referenceConstraintAlgorithm)
        referenceConstraintAlgorithm->apply(atomCoordinates, xPrime, inverseMasses, tolerance);

   // Update the positions and velocities.

   RealOpenMM velocityScale = one/getDeltaT();
   for (int i = 0; i < numberOfAtoms; ++i) {
       if (masses[i] != zero)
           for (int j = 0; j < 3; ++j) {
               velocities[i][j] = velocityScale*(xPrime[i][j] - atomCoordinates[i][j]);
               atomCoordinates[i][j] = xPrime[i][j];
           }
   }
   ReferenceVirtualSites::computePositions(system, atomCoordinates);
   incrementTimeStep();
}