ljcos.hpp

Go to the documentation of this file.

/*
 * Copyright (C) 2010-2022 The ESPResSo project
 * Copyright (C) 2002,2003,2004,2005,2006,2007,2008,2009,2010
 *   Max-Planck-Institute for Polymer Research, Theory Group
 *
 * This file is part of ESPResSo.
 *
 * ESPResSo is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * ESPResSo is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
#pragma once
 
/** \file
 *  Routines to calculate the Lennard-Jones+cosine potential between
 *  particle pairs.
 *
 *  Implementation in \ref ljcos.cpp.
 */
 
#include "config/config.hpp"
 
#ifdef ESPRESSO_LJCOS
 
#include "nonbonded_interaction_data.hpp"
 
#include <utils/math/int_pow.hpp>
#include <utils/math/sqr.hpp>
 
#include <cmath>
 
/** Calculate Lennard-Jones cosine force factor */
inline double ljcos_pair_force_factor(IA_parameters const &ia_params,
                                      double dist) {
  auto fac = 0.0;
  if (dist < (ia_params.ljcos.cut + ia_params.ljcos.offset)) {
    auto const r_off = dist - ia_params.ljcos.offset;
    /* cos part of ljcos potential. */
    if (dist > ia_params.ljcos.rmin + ia_params.ljcos.offset) {
      fac = (r_off / dist) * ia_params.ljcos.alfa * ia_params.ljcos.eps *
            (sin(ia_params.ljcos.alfa * Utils::sqr(r_off) +
                 ia_params.ljcos.beta));
    }
    /* Lennard-Jones part of the potential. */
    else if (dist > 0) {
      auto const frac6 = Utils::int_pow<6>(ia_params.ljcos.sig / r_off);
      fac = 48.0 * ia_params.ljcos.eps * frac6 * (frac6 - 0.5) / (r_off * dist);
    }
  }
  return fac;
}
 
/** Calculate Lennard-Jones cosine energy */
inline double ljcos_pair_energy(IA_parameters const &ia_params, double dist) {
  if (dist < (ia_params.ljcos.cut + ia_params.ljcos.offset)) {
    auto const r_off = dist - ia_params.ljcos.offset;
    /* Lennard-Jones part of the potential. */
    if (dist < (ia_params.ljcos.rmin + ia_params.ljcos.offset)) {
      auto const frac6 = Utils::int_pow<6>(ia_params.ljcos.sig / r_off);
      return 4.0 * ia_params.ljcos.eps * (Utils::sqr(frac6) - frac6);
    }
    /* cosine part of the potential. */
    return .5 * ia_params.ljcos.eps *
           (cos(ia_params.ljcos.alfa * Utils::sqr(r_off) +
                ia_params.ljcos.beta) -
            1.);
  }
  return 0.0;
}
 
#endif // ESPRESSO_LJCOS