HarmonicTorsionType.hpp

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 * [1] Meineke, et al., J. Comp. Chem. 26, 252-271 (2005).
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 * [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).
 */
 
#ifndef TYPES_HARMONICTORSIONTYPE_HPP
#define TYPES_HARMONICTORSIONTYPE_HPP
 
#if defined(_MSC_VER)
#define copysign _copysign
#endif
 
namespace OpenMD {
 
  /**
   * @class HarmonicTorsionType
   * These torsion types are defined identically with functional form given
   * in equation 5 in the following paper:
   *
   * "Transferable Potentials for Phase Equilibria. 4. United-Atom
   * Description of Linear and Branched Alkenes and Alkylbenzenes" by
   * Collin D. Wick, Marcus G. Martin and J. Ilja Siepmann,
   * J. Phys. Chem. B; 2000; 104(33) pp 8008 - 8016;
   *
   *    http://pubs.acs.org/doi/abs/10.1021/jp001044x
   *
   * This torsion potential has the form:
   *
   *  \f[
         V_{tors} = \frac{d_0}{2} \left(\phi - \phi_0\right)^2
       \f]
   *
   */
  class HarmonicTorsionType : public TorsionType {
  public:
    HarmonicTorsionType(RealType d0, RealType phi0) :
        TorsionType(), d0_(d0), phi0_(phi0) {}
 
    virtual void calcForce(RealType cosPhi, RealType& V, RealType& dVdCosPhi) {
      // check roundoff
      if (cosPhi > 1.0) {
        cosPhi = 1.0;
      } else if (cosPhi < -1.0) {
        cosPhi = -1.0;
      }
 
      RealType phi    = acos(cosPhi);
      RealType sinPhi = sqrt(1.0 - cosPhi * cosPhi);
 
      // trick to avoid divergence in angles near 0 and pi:
 
      if (fabs(sinPhi) < 1.0E-6) { sinPhi = copysign(1.0E-6, sinPhi); }
 
      V = 0.5 * d0_ * pow((phi - phi0_), 2);
 
      dVdCosPhi = -d0_ * (phi - phi0_) / sinPhi;
    }
 
    friend std::ostream& operator<<(std::ostream& os, HarmonicTorsionType& ttt);
 
  private:
    RealType d0_;
    RealType phi0_;
  };
 
  std::ostream& operator<<(std::ostream& os, HarmonicTorsionType& htt) {
    os << "This HarmonicTorsionType has below form:" << std::endl;
    os << htt.d0_ << "*(phi - " << htt.phi0_ << ")/2" << std::endl;
    return os;
  }
 
}  // namespace OpenMD
 
#endif  // TYPES_HARMONICTORSIONTYPE_HPP