FluctuatingChargeLangevin.cpp

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 * SUPPORT OPEN SCIENCE!  If you use OpenMD or its source code in your
 * research, please cite the following paper when you publish your work:
 *
 * [1] Drisko et al., J. Open Source Softw. 9, 7004 (2024).
 *
 * Good starting points for code and simulation methodology are:
 *
 * [2] Meineke, et al., J. Comp. Chem. 26, 252-271 (2005).
 * [3] Fennell & Gezelter, J. Chem. Phys. 124, 234104 (2006).
 * [4] Sun, Lin & Gezelter, J. Chem. Phys. 128, 234107 (2008).
 * [5] Vardeman, Stocker & Gezelter, J. Chem. Theory Comput. 7, 834 (2011).
 * [6] Kuang & Gezelter, Mol. Phys., 110, 691-701 (2012).
 * [7] Lamichhane, Gezelter & Newman, J. Chem. Phys. 141, 134109 (2014).
 * [8] Bhattarai, Newman & Gezelter, Phys. Rev. B 99, 094106 (2019).
 * [9] Drisko & Gezelter, J. Chem. Theory Comput. 20, 4986-4997 (2024).
 */
 
#include "FluctuatingChargeLangevin.hpp"
 
#include <cmath>
 
#include "primitives/Molecule.hpp"
#include "utils/Constants.hpp"
#include "utils/simError.h"
 
namespace OpenMD {
 
  FluctuatingChargeLangevin::FluctuatingChargeLangevin(SimInfo* info) :
      FluctuatingChargePropagator(info), maxIterNum_(4), forceTolerance_(1e-6),
      snap_(info->getSnapshotManager()->getCurrentSnapshot()),
      randNumGen_(info->getRandomNumberGenerator()) {}
 
  void FluctuatingChargeLangevin::initialize() {
    FluctuatingChargePropagator::initialize();
    if (hasFlucQ_) {
      if (info_->getSimParams()->haveDt()) {
        dt_  = info_->getSimParams()->getDt();
        dt2_ = dt_ * 0.5;
      } else {
        snprintf(painCave.errMsg, MAX_SIM_ERROR_MSG_LENGTH,
                 "FluctuatingChargeLangevin Error: dt is not set\n");
        painCave.isFatal = 1;
        simError();
      }
 
      if (!fqParams_->haveTargetTemp()) {
        snprintf(painCave.errMsg, MAX_SIM_ERROR_MSG_LENGTH,
                 "You can't use the FluctuatingChargeLangevin "
                 "propagator without a flucQ.targetTemp!\n");
        painCave.isFatal  = 1;
        painCave.severity = OPENMD_ERROR;
        simError();
      } else {
        targetTemp_ = fqParams_->getTargetTemp();
      }
 
      // We must set tauThermostat.
 
      if (!fqParams_->haveDragCoefficient()) {
        snprintf(painCave.errMsg, MAX_SIM_ERROR_MSG_LENGTH,
                 "If you use the FluctuatingChargeLangevin\n"
                 "\tpropagator, you must set flucQ.dragCoefficient .\n");
 
        painCave.severity = OPENMD_ERROR;
        painCave.isFatal  = 1;
        simError();
      } else {
        drag_ = fqParams_->getDragCoefficient();
      }
    }
 
    RealType stdDev =
        std::sqrt(2.0 * Constants::kb * targetTemp_ * drag_ / dt_);
 
    forceDistribution_ = std::normal_distribution<RealType>(0.0, stdDev);
  }
 
  void FluctuatingChargeLangevin::moveA() {
    if (!hasFlucQ_) return;
 
    SimInfo::MoleculeIterator i;
    Molecule::FluctuatingChargeIterator j;
    Molecule* mol;
    Atom* atom;
    RealType cvel, cpos, cfrc, cmass;
 
    for (mol = info_->beginMolecule(i); mol != NULL;
         mol = info_->nextMolecule(i)) {
      for (atom = mol->beginFluctuatingCharge(j); atom != NULL;
           atom = mol->nextFluctuatingCharge(j)) {
        cvel  = atom->getFlucQVel();
        cpos  = atom->getFlucQPos();
        cfrc  = atom->getFlucQFrc();
        cmass = atom->getChargeMass();
 
        // velocity half step
        cvel += dt2_ * cfrc / cmass;
        // position whole step
        cpos += dt_ * cvel;
 
        atom->setFlucQVel(cvel);
        atom->setFlucQPos(cpos);
      }
    }
  }
 
  void FluctuatingChargeLangevin::applyConstraints() {
    if (!hasFlucQ_) return;
 
    SimInfo::MoleculeIterator i;
    Molecule::FluctuatingChargeIterator j;
    Molecule* mol;
    Atom* atom;
    RealType cvel, cfrc, cmass, randomForce, frictionForce;
    RealType velStep, oldFF;  // used to test for convergence
 
    for (mol = info_->beginMolecule(i); mol != NULL;
         mol = info_->nextMolecule(i)) {
      for (atom = mol->beginFluctuatingCharge(j); atom != NULL;
           atom = mol->nextFluctuatingCharge(j)) {
        randomForce = forceDistribution_(*randNumGen_);
        atom->addFlucQFrc(randomForce);
 
        // What remains contains velocity explicitly, but the velocity
        // required is at the full step: v(t + h), while we have
        // initially the velocity at the half step: v(t + h/2).  We
        // need to iterate to converge the friction force vector.
 
        // this is the velocity at the half-step:
 
        cvel = atom->getFlucQVel();
 
        // estimate velocity at full-step using everything but
        // friction forces:
 
        cfrc    = atom->getFlucQFrc();
        cmass   = atom->getChargeMass();
        velStep = cvel + dt2_ * cfrc / cmass;
 
        frictionForce = 0.0;
 
        // iteration starts here:
 
        for (int k = 0; k < maxIterNum_; k++) {
          oldFF         = frictionForce;
          frictionForce = -drag_ * velStep;
          // re-estimate velocities at full-step using friction forces:
 
          velStep = cvel + dt2_ * (cfrc + frictionForce) / cmass;
 
          // check for convergence
 
          if (fabs(frictionForce - oldFF) <= forceTolerance_)
            break;  // iteration ends here
        }
        atom->addFlucQFrc(frictionForce);
      }
    }
    fqConstraints_->applyConstraints();
  }
 
  void FluctuatingChargeLangevin::moveB() {
    if (!hasFlucQ_) return;
    SimInfo::MoleculeIterator i;
    Molecule::FluctuatingChargeIterator j;
    Molecule* mol;
    Atom* atom;
    RealType cfrc, cvel, cmass;
 
    for (mol = info_->beginMolecule(i); mol != NULL;
         mol = info_->nextMolecule(i)) {
      for (atom = mol->beginFluctuatingCharge(j); atom != NULL;
           atom = mol->nextFluctuatingCharge(j)) {
        cvel  = atom->getFlucQVel();
        cfrc  = atom->getFlucQFrc();
        cmass = atom->getChargeMass();
 
        // velocity half step
        cvel += (dt2_ * cfrc) / cmass;
 
        atom->setFlucQVel(cvel);
      }
    }
  }
}  // namespace OpenMD