bonded_coulomb.hpp

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/*
 * 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 bonded Coulomb potential between
 *  particle pairs.
 */
 
#include "config/config.hpp"
 
#include <utils/Vector.hpp>
 
#include <cmath>
#include <optional>
 
/** Parameters for Coulomb bond Potential */
struct BondedCoulomb {
  /** Coulomb prefactor */
  double prefactor;
 
  double cutoff() const { return 0.; }
 
  static constexpr int num = 1;
 
  BondedCoulomb(double prefactor) { this->prefactor = prefactor; }
 
  std::optional<Utils::Vector3d> force(double q1q2,
                                       Utils::Vector3d const &dx) const;
  std::optional<double> energy(double q1q2, Utils::Vector3d const &dx) const;
};
 
/** Compute the bonded Coulomb pair force.
 *  @param[in]  q1q2      Product of the particle charges.
 *  @param[in]  dx        Distance between the particles.
 */
inline std::optional<Utils::Vector3d>
BondedCoulomb::force(double const q1q2, Utils::Vector3d const &dx) const {
#ifdef ELECTROSTATICS
  auto const dist2 = dx.norm2();
  auto const dist3 = dist2 * std::sqrt(dist2);
  auto const fac = prefactor * q1q2 / dist3;
  return fac * dx;
#else
  return Utils::Vector3d{};
#endif
}
 
/** Compute the bonded Coulomb pair energy.
 *  @param[in]  q1q2      Product of the particle charges.
 *  @param[in]  dx        Distance between the particles.
 */
inline std::optional<double>
BondedCoulomb::energy(double const q1q2, Utils::Vector3d const &dx) const {
#ifdef ELECTROSTATICS
  auto const dist = dx.norm();
  return prefactor * q1q2 / dist;
#else
  return .0;
#endif
}