Bend.cpp

/*
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 *
 * SUPPORT OPEN SCIENCE!  If you use OpenMD or its source code in your
 * research, please cite the appropriate papers when you publish your
 * work.  Good starting points are:
 *
 * [1] Meineke, et al., J. Comp. Chem. 26, 252-271 (2005).
 * [2] Fennell & Gezelter, J. Chem. Phys. 124, 234104 (2006).
 * [3] Sun, Lin & Gezelter, J. Chem. Phys. 128, 234107 (2008).
 * [4] Vardeman, Stocker & Gezelter, J. Chem. Theory Comput. 7, 834 (2011).
 * [5] Kuang & Gezelter, Mol. Phys., 110, 691-701 (2012).
 * [6] Lamichhane, Gezelter & Newman, J. Chem. Phys. 141, 134109 (2014).
 * [7] Lamichhane, Newman & Gezelter, J. Chem. Phys. 141, 134110 (2014).
 * [8] Bhattarai, Newman & Gezelter, Phys. Rev. B 99, 094106 (2019).
 */
 
#include "primitives/Bend.hpp"
 
#include <config.h>
 
#include <cmath>
 
#include "utils/Constants.hpp"
 
namespace OpenMD {
 
  /**@todo still a lot left to improve*/
  void Bend::calcForce(RealType& angle, bool doParticlePot) {
    Vector3d pos1 = atoms_[0]->getPos();
    Vector3d pos2 = atoms_[1]->getPos();
    Vector3d pos3 = atoms_[2]->getPos();
 
    Vector3d r21 = pos1 - pos2;
    snapshotMan_->getCurrentSnapshot()->wrapVector(r21);
    RealType d21 = r21.length();
 
    RealType d21inv = 1.0 / d21;
 
    Vector3d r23 = pos3 - pos2;
    snapshotMan_->getCurrentSnapshot()->wrapVector(r23);
    RealType d23 = r23.length();
 
    RealType d23inv = 1.0 / d23;
 
    RealType cosTheta = dot(r21, r23) / (d21 * d23);
 
    // check roundoff
    if (cosTheta > 1.0) {
      cosTheta = 1.0;
    } else if (cosTheta < -1.0) {
      cosTheta = -1.0;
    }
 
    RealType theta = acos(cosTheta);
 
    RealType dVdTheta;
 
    bendType_->calcForce(theta, potential_, dVdTheta);
 
    RealType sinTheta = sqrt(1.0 - cosTheta * cosTheta);
 
    if (fabs(sinTheta) < 1.0E-6) { sinTheta = 1.0E-6; }
 
    RealType commonFactor1 = dVdTheta / sinTheta * d21inv;
    RealType commonFactor2 = dVdTheta / sinTheta * d23inv;
 
    Vector3d force1 = commonFactor1 * (r23 * d23inv - r21 * d21inv * cosTheta);
    Vector3d force3 = commonFactor2 * (r21 * d21inv - r23 * d23inv * cosTheta);
 
    // Total force in current bend is zero
    Vector3d force2 = force1 + force3;
    force2 *= -1.0;
 
    atoms_[0]->addFrc(force1);
    atoms_[1]->addFrc(force2);
    atoms_[2]->addFrc(force3);
 
    if (doParticlePot) {
      atoms_[0]->addParticlePot(potential_);
      atoms_[1]->addParticlePot(potential_);
      atoms_[2]->addParticlePot(potential_);
    }
 
    angle = theta / Constants::PI * 180.0;
  }
 
}  // namespace OpenMD