118 lines
3 KiB
Text
118 lines
3 KiB
Text
#ifndef POTENTIALS_CUH
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#define POTENTIALS_CUH
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#include "precision.hpp"
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#include "vec3.h"
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#include <cmath>
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#include <cstdio>
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#include <variant>
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#ifdef __CUDACC__
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#define CUDA_CALLABLE __host__ __device__
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#else
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#define CUDA_CALLABLE
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#endif
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/**
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* Result struct for the Pair Potential
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*/
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struct ForceAndEnergy {
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real energy;
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Vec3<real> force;
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CUDA_CALLABLE inline static ForceAndEnergy zero() {
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return {0.0, {0.0, 0.0, 0.0}};
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};
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};
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/**
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* Calculate the Lennard-Jones energy and force for the current particle
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* pair described by displacement vector r
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*/
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struct LennardJones {
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real m_sigma;
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real m_epsilon;
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real m_rcutoffsq;
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CUDA_CALLABLE LennardJones(real sigma, real epsilon, real rcutoff) {
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m_sigma = sigma;
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m_epsilon = epsilon;
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m_rcutoffsq = rcutoff * rcutoff;
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};
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CUDA_CALLABLE ForceAndEnergy calc_force_and_energy(Vec3<real> r) {
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real rmagsq = r.squared_norm2();
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if (rmagsq < m_rcutoffsq && rmagsq > 0.0) {
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real inv_rmag = 1 / sqrt(rmagsq);
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// Pre-Compute the terms (doing this saves on multiple devisions/pow
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// function call)
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real sigma_r = m_sigma * inv_rmag;
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real sigma_r6 = sigma_r * sigma_r * sigma_r * sigma_r * sigma_r * sigma_r;
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real sigma_r12 = sigma_r6 * sigma_r6;
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// Get the energy
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real energy = 4.0 * m_epsilon * (sigma_r12 - sigma_r6);
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// Get the force vector
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real force_mag =
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4.0 * m_epsilon *
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(12.0 * sigma_r12 * inv_rmag - 6.0 * sigma_r6 * inv_rmag);
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Vec3<real> force = r.scale(force_mag * inv_rmag);
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return {energy, force};
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} else {
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return ForceAndEnergy::zero();
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}
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};
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};
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/**
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* Calculate the Morse potential energy and force for the current particle pair
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* described by displacement vector r
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*/
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struct Morse {
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real m_D; // Depth of the potential well
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real m_a; // Width of the potential
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real m_r0; // Equilibrium bond distance
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real m_rcutoffsq; // Cutoff distance squared
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CUDA_CALLABLE Morse(real D, real a, real r0, real rcutoff) {
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m_D = D;
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m_a = a;
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m_r0 = r0;
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m_rcutoffsq = rcutoff * rcutoff;
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};
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CUDA_CALLABLE ForceAndEnergy calc_force_and_energy(Vec3<real> r) {
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real rmagsq = r.squared_norm2();
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if (rmagsq < m_rcutoffsq && rmagsq > 0.0) {
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real rmag = sqrt(rmagsq);
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real dr = rmag - m_r0;
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// Compute exponentials
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real exp_a_dr = exp(-m_a * dr);
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real exp_2a_dr = exp_a_dr * exp_a_dr;
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// Energy: V(r) = D * (exp(-2a(r - r0)) - 2*exp(-a(r - r0)))
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real energy = m_D * (exp_2a_dr - 2.0 * exp_a_dr);
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// Force magnitude: F(r) = 2aD * (exp(-2a(r - r0)) - exp(-a(r - r0)))
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real force_mag = 2.0 * m_a * m_D * (exp_2a_dr - exp_a_dr);
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// Direction: normalized vector
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Vec3<real> force = r.scale(force_mag / rmag);
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return {energy, force};
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} else {
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return ForceAndEnergy::zero();
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}
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};
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};
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// Variant type for storing pair potential types
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using PairPotentials = std::variant<LennardJones, Morse>;
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#endif
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