text: // Copyright (c) 2009-2017 The Regents of the University of Michigan
  // This file is part of the HOOMD-blue project, released under the BSD 3-Clause License.
  
  
  // Maintainer: jglaser
  
  #ifndef __PAIR_EVALUATOR_REACTION_FIELD_H__
  #define __PAIR_EVALUATOR_REACTION_FIELD_H__
  
  #ifndef NVCC
  #include <string>
  #endif
  
  #include "hoomd/HOOMDMath.h"
  
  /*! \file EvaluatorPairReactionField.h
      \brief Defines the pair evaluator class for ReactionField potentials
  */
  
  // need to declare these class methods with __device__ qualifiers when building in nvcc
  // DEVICE is __host__ __device__ when included in nvcc and blank when included into the host compiler
  #ifdef NVCC
  #define DEVICE __device__
  #else
  #define DEVICE
  #endif
  
  //! Class for evaluating the Onsager reaction field pair potential
  /*! <b>General Overview</b>
  
      See EvaluatorPairLJ
  
      <b>ReactionField specifics</b>
  
      EvaluatorPairReactionField evaluates the function:
      \f[ V_{\mathrm{RF}}(r) = \varepsilon \left[ \frac{1}{r} +
          \frac{(\epsilon_{RF}-1) r^2}{(2 \epsilon_{RF} + 1) r_c^3} \right]\f]
  
      The reaction field potential does not require charge or diameter. Two parameters,
      \f$ \varepsilon \f$ and \f$ \epsilon_{RF} \f$ are needed.
  
      \a \varepsilon is placed in \a params.x and \a \epsilon_{RF} is in \a params.y.
  
      If \epsilon_{RF} is zero, it will be treated as infinity.
  */
  class EvaluatorPairReactionField
      {
      public:
          //! Define the parameter type used by this pair potential evaluator
          typedef Scalar3 param_type;
  
          //! Constructs the pair potential evaluator
          /*! \param _rsq Squared distance beteen the particles
              \param _rcutsq Squared distance at which the potential goes to 0
              \param _params Per type pair parameters of this potential
          */
          DEVICE EvaluatorPairReactionField(Scalar _rsq, Scalar _rcutsq, const param_type& _params)
              : rsq(_rsq), rcutsq(_rcutsq), epsilon(_params.x), epsrf(_params.y), use_charge(__scalar_as_int(_params.z)), qiqj(1.0)
              {
              }
  
          //! ReactionField doesn't use diameter
          DEVICE static bool needsDiameter() { return false; }
          //! Accept the optional diameter values
          /*! \param di Diameter of particle i
              \param dj Diameter of particle j
          */
          DEVICE void setDiameter(Scalar di, Scalar dj) { }
  
          //! ReactionField uses charge
          DEVICE static bool needsCharge() { return true; }
  
          //! Accept the optional charge values
          /*! \param qi Charge of particle i
              \param qj Charge of particle j
          */
          DEVICE void setCharge(Scalar qi, Scalar qj)
              {
              if (use_charge) qiqj = qi*qj;
              }
  
          //! Evaluate the force and energy
          /*! \param force_divr Output parameter to write the computed force divided by r.
              \param pair_eng Output parameter to write the computed pair energy
              \param energy_shift If true, the potential must be shifted so that V(r) is continuous at the cutoff
              \note There is no need to check if rsq < rcutsq in this method. Cutoff tests are performed
                    in PotentialPair.
  
              \return True if they are evaluated or false if they are not because we are beyond the cuttoff
          */
          DEVICE bool evalForceAndEnergy(Scalar& force_divr, Scalar& pair_eng, bool energy_shift)
              {
              // compute the force divided by r in force_divr
              if (rsq < rcutsq && epsilon != 0 && qiqj != 0)
                  {
                  Scalar rcut3inv = fast::rsqrt(rcutsq)/rcutsq;
                  Scalar rinv = fast::rsqrt(rsq);
                  Scalar r = Scalar(1.0) / rinv;
                  Scalar r2inv = Scalar(1.0) / rsq;
  
                  Scalar eps_fac = (epsrf - Scalar(1.0))/(Scalar(2.0)*epsrf+Scalar(1.0))*rcut3inv;
                  if (epsrf == Scalar(0.0))
                      {
                      eps_fac = Scalar(1.0/2.0)*rcut3inv;
                      }
  
                  force_divr = qiqj*epsilon * (r2inv * rinv - Scalar(2.0)*eps_fac);
                  pair_eng = qiqj*epsilon * (rinv + eps_fac*r*r);
  
                  if (energy_shift)
                      {
                      Scalar rcutinv = fast::rsqrt(rcutsq);
                      Scalar rcut = Scalar(1.0) / rcutinv;
                      pair_eng -= qiqj*epsilon * (rcutinv + eps_fac*rcut*rcut);
                      }
                  return true;
                  }
              else
                  return false;
              }
  
          #ifndef NVCC
          //! Get the name of this potential
          /*! \returns The potential name. Must be short and all lowercase, as this is the name energies will be logged as
              via analyze.log.
          */
          static std::string getName()
              {
              return std::string("reaction_field");
              }
          #endif
  
      protected:
          Scalar rsq;     //!< Stored rsq from the constructor
          Scalar rcutsq;  //!< Stored rcutsq from the constructor
          Scalar epsilon; //!< epsilon parameter extracted from the params passed to the constructor
          Scalar epsrf;   //!< epsilon_rf parameter extracted from the params passed to the constructor
          bool use_charge; //!< True if we are using the particle charges
          Scalar qiqj;    //!< Product of charges
      };
  
  
  #endif // __PAIR_EVALUATOR_REACTION_FIELD_H__
  