MorseBondType.hpp

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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).
 */
 
#ifndef TYPES_MORSEBONDTYPE_HPP
#define TYPES_MORSEBONDTYPE_HPP
 
#include <cmath>
 
#include "types/BondType.hpp"
 
namespace OpenMD {
 
  /**
 * @class MorseBondType
 *
 * @brief MorseBondType is a more realistic bond potential.
 *
 * The functional form is given by:
   \f[
      V(r) = D_e (1 - e^{-\beta (r - r_0)})^2
    \f]
 * where \f$D_e\f$ is the bond dissociation energy (in
 * kcal / mol), \f$\beta\f$ is an inverse distance parameter related
 * to the force constant. \f$\beta = \sqrt{\frac{k}{2 D_e}}\f$, and
 * \f$r_0\f$ is the equilibrium bond length.
 */
  class MorseBondType : public BondType {
  public:
    MorseBondType(RealType myR0, RealType myD, RealType myBeta) :
        BondType(myR0), De(myD), beta(myBeta) {}
 
    void setWellDepth(RealType myD) { De = myD; }
 
    void setBeta(RealType myBeta) { beta = myBeta; }
 
    void setWellDepthAndForceConstant(RealType myD, RealType myK) {
      De   = myD;
      beta = sqrt(myK / (2.0 * De));
    }
 
    RealType getWellDepth() { return De; }
 
    RealType getBeta() { return beta; }
 
    RealType getForceConstant() { return 2.0 * De * beta * beta; }
 
    virtual void calcForce(RealType r, RealType& V, RealType& dVdr) {
      RealType dr, eterm, eterm2;
 
      dr     = r - r0;
      eterm  = exp(-beta * dr);
      eterm2 = eterm * eterm;
 
      V    = De * (1 - 2.0 * eterm + eterm2);
      dVdr = 2.0 * De * beta * (eterm - eterm2);
    }
 
  private:
    RealType De;
    RealType beta;
  };
}  // namespace OpenMD
 
#endif