CosineSeriesBendType.hpp

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 * [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).
 */
 
#ifndef TYPES_COSINESERIESBENDTYPE_HPP
#define TYPES_COSINESERIESBENDTYPE_HPP
 
#include "types/BendType.hpp"
 
namespace OpenMD {
  /**
   * @class CosineSeriesBendType
   *
   * A bend used primarily in UFF that utilizes a Fourier series in
   * terms of the cosine of the bend angle:
   *
   * \f[ Vbend = k \left[ C_0 + C_1 \cos(\theta) + C_2 \cos(2 \theta) \right]
   * \f]
   */
  class CosineSeriesBendType : public BendType {
  public:
    CosineSeriesBendType(RealType theta, RealType k) : BendType(theta), k_(k) {
      RealType st = sin(theta);
      RealType ct = cos(theta);
      c2_         = 1.0 / (4.0 * st * st);
      c1_         = -4.0 * c2_ * ct;
      c0_         = c2_ * (2.0 * ct * ct + 1.0);
    }
 
    void setForceConstant(RealType k) { k_ = k; }
 
    RealType getForceConstant() { return k_; }
 
    void calcForce(RealType theta, RealType& V, RealType& dVdtheta) {
      RealType ct = cos(theta);
      RealType st = sin(theta);
 
      // use cos 2t = (2 cos^2 - 1)
      V        = k_ * (c0_ + c1_ * ct + c2_ * (2.0 * ct * ct - 1.0));
      dVdtheta = -k_ * (c1_ * st + 2.0 * c2_ * sin(2.0 * theta));
    }
 
  private:
    RealType k_;
    RealType c0_;
    RealType c1_;
    RealType c2_;
  };
}  // namespace OpenMD
 
#endif  // TYPES_COSINESERIESBENDTYPE_HPP