FluctuatingChargeNVT.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 "FluctuatingChargeNVT.hpp"
 
#include "primitives/Molecule.hpp"
#include "utils/Constants.hpp"
#include "utils/simError.h"
 
namespace OpenMD {
 
  FluctuatingChargeNVT::FluctuatingChargeNVT(SimInfo* info) :
      FluctuatingChargePropagator(info), maxIterNum_(4), chiTolerance_(1e-6),
      snap(info->getSnapshotManager()->getCurrentSnapshot()), thermo(info) {}
 
  void FluctuatingChargeNVT::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,
                 "FluctuatingChargeNVT Error: dt is not set\n");
        painCave.isFatal = 1;
        simError();
      }
 
      if (!info_->getSimParams()->getUseIntialExtendedSystemState()) {
        snap->setElectronicThermostat(make_pair(0.0, 0.0));
      }
 
      if (!fqParams_->haveTargetTemp()) {
        snprintf(painCave.errMsg, MAX_SIM_ERROR_MSG_LENGTH,
                 "You can't use the FluctuatingChargeNVT "
                 "propagator without a flucQ.targetTemp!\n");
        painCave.isFatal  = 1;
        painCave.severity = OPENMD_ERROR;
        simError();
      } else {
        targetTemp_ = fqParams_->getTargetTemp();
      }
 
      // We must set tauThermostat.
 
      if (!fqParams_->haveTauThermostat()) {
        snprintf(painCave.errMsg, MAX_SIM_ERROR_MSG_LENGTH,
                 "If you use the FluctuatingChargeNVT\n"
                 "\tpropagator, you must set flucQ.tauThermostat .\n");
 
        painCave.severity = OPENMD_ERROR;
        painCave.isFatal  = 1;
        simError();
      } else {
        tauThermostat_ = fqParams_->getTauThermostat();
      }
      updateSizes();
    }
  }
 
  void FluctuatingChargeNVT::moveA() {
    if (!hasFlucQ_) return;
 
    SimInfo::MoleculeIterator i;
    Molecule::FluctuatingChargeIterator j;
    Molecule* mol;
    Atom* atom;
    RealType cvel, cpos, cfrc, cmass;
 
    pair<RealType, RealType> thermostat = snap->getElectronicThermostat();
    RealType chi                        = thermostat.first;
    RealType integralOfChidt            = thermostat.second;
    RealType instTemp                   = thermo.getElectronicTemperature();
 
    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 - dt2_ * chi * cvel;
        // position whole step
        cpos += dt_ * cvel;
 
        atom->setFlucQVel(cvel);
        atom->setFlucQPos(cpos);
      }
    }
 
    chi += dt2_ * (instTemp / targetTemp_ - 1.0) /
           (tauThermostat_ * tauThermostat_);
 
    integralOfChidt += chi * dt2_;
    snap->setElectronicThermostat(make_pair(chi, integralOfChidt));
  }
 
  void FluctuatingChargeNVT::updateSizes() {
    oldVel_.resize(info_->getNFluctuatingCharges());
  }
 
  void FluctuatingChargeNVT::moveB() {
    if (!hasFlucQ_) return;
    SimInfo::MoleculeIterator i;
    Molecule::FluctuatingChargeIterator j;
    Molecule* mol;
    Atom* atom;
    RealType instTemp;
    pair<RealType, RealType> thermostat = snap->getElectronicThermostat();
    RealType chi                        = thermostat.first;
    RealType oldChi                     = chi;
    RealType prevChi;
    RealType integralOfChidt = thermostat.second;
    int index;
    RealType cfrc, cvel, cmass;
 
    index = 0;
    for (mol = info_->beginMolecule(i); mol != NULL;
         mol = info_->nextMolecule(i)) {
      for (atom = mol->beginFluctuatingCharge(j); atom != NULL;
           atom = mol->nextFluctuatingCharge(j)) {
        oldVel_[index] = atom->getFlucQVel();
        ++index;
      }
    }
 
    // do the iteration:
 
    for (int k = 0; k < maxIterNum_; k++) {
      index    = 0;
      instTemp = thermo.getElectronicTemperature();
      // evolve chi another half step using the temperature at t + dt/2
      prevChi = chi;
      chi     = oldChi + dt2_ * (instTemp / targetTemp_ - 1.0) /
                         (tauThermostat_ * tauThermostat_);
 
      for (mol = info_->beginMolecule(i); mol != NULL;
           mol = info_->nextMolecule(i)) {
        for (atom = mol->beginFluctuatingCharge(j); atom != NULL;
             atom = mol->nextFluctuatingCharge(j)) {
          cfrc  = atom->getFlucQFrc();
          cmass = atom->getChargeMass();
 
          // velocity half step
          cvel = oldVel_[index] + dt2_ * cfrc / cmass -
                 dt2_ * chi * oldVel_[index];
          atom->setFlucQVel(cvel);
          ++index;
        }
      }
      if (fabs(prevChi - chi) <= chiTolerance_) break;
    }
    integralOfChidt += dt2_ * chi;
    snap->setElectronicThermostat(make_pair(chi, integralOfChidt));
  }
 
  void FluctuatingChargeNVT::resetPropagator() {
    if (!hasFlucQ_) return;
    snap->setElectronicThermostat(make_pair(0.0, 0.0));
  }
 
  RealType FluctuatingChargeNVT::calcConservedQuantity() {
    if (!hasFlucQ_) return 0.0;
    pair<RealType, RealType> thermostat = snap->getElectronicThermostat();
    RealType chi                        = thermostat.first;
    RealType integralOfChidt            = thermostat.second;
    RealType fkBT =
        info_->getNFluctuatingCharges() * Constants::kB * targetTemp_;
 
    RealType thermostat_kinetic = fkBT * tauThermostat_ * tauThermostat_ * chi *
                                  chi / (2.0 * Constants::energyConvert);
 
    RealType thermostat_potential =
        fkBT * integralOfChidt / Constants::energyConvert;
 
    return thermostat_kinetic + thermostat_potential;
  }
}  // namespace OpenMD