Add basic LJ potential*

- Add PairPotential Abstract class
- Add Lennard-Jones potential that should work with both CUDA and C++
  code
- Add tests on HOST side for LJ potential

src/CMakeLists.txt

@@ -4,14 +4,13 @@ set(HEADER_FILES
     particle.hpp
     simulation.hpp
     box.hpp
-    pair_potentials.hpp
+
 )
 set(SOURCE_FILES
-    pair_potentials.cpp
 )
 
 # The library contains header and source files.
-add_library(${NAME}_lib 
+add_library(${NAME}_lib INTERFACE
     ${HEADER_FILES} 
     ${SOURCE_FILES}
 )

src/pair_potentials.cpp

@@ -1,33 +0,0 @@
-#include "pair_potentials.hpp"
-#include <cmath>
-
-PairPotential::~PairPotential() {};
-/**
- * Calculate the Lennard-Jones energy and force for the current particle pair
- * described by displacement vector r
- */
-ForceAndEnergy LennardJones::calc_force_and_energy(Vec3<real> r) {
-  real rmagsq = r.squared_norm2();
-  if (rmagsq < this->m_rcutoffsq && rmagsq > 0.0) {
-    real inv_rmag = 1 / std::sqrt(rmagsq);
-
-    // Pre-Compute the terms (doing this saves on multiple devisions/pow
-    // function call)
-    real sigma_r = m_sigma * inv_rmag;
-    real sigma_r6 = sigma_r * sigma_r * sigma_r * sigma_r * sigma_r * sigma_r;
-    real sigma_r12 = sigma_r6 * sigma_r6;
-
-    // Get the energy
-    real energy = 4.0 * m_epsilon * (sigma_r12 - sigma_r6);
-
-    // Get the force vector
-    real force_mag = 4.0 * m_epsilon *
-                     (12.0 * sigma_r12 * inv_rmag - 6.0 * sigma_r6 * inv_rmag);
-    Vec3<real> force = r.scale(force_mag * inv_rmag);
-
-    return {energy, force};
-
-  } else {
-    return ForceAndEnergy::zero();
-  }
-};

src/pair_potentials.hpp

@@ -1,49 +0,0 @@
-#ifndef POTENTIALS_H
-#define POTENTIALS_H
-
-#include "precision.hpp"
-#include "vec3.h"
-
-/**
- * Result struct for the Pair Potential
- */
-struct ForceAndEnergy {
-  real energy;
-  Vec3<real> force;
-
-  inline static ForceAndEnergy zero() { return {0.0, {0.0, 0.0, 0.0}}; };
-};
-
-/**
- * Abstract implementation of a Pair Potential.
- * Pair potentials are potentials which depend solely on the distance
- * between two particles. These do not include multi-body potentials such as
- * EAM
- *
- */
-struct PairPotential {
-  real m_rcutoffsq;
-
-  PairPotential(real rcutoff) : m_rcutoffsq(rcutoff * rcutoff) {};
-  virtual ~PairPotential() = 0;
-
-  /**
-   * Calculate the force and energy for a specific atom pair based on a
-   * displacement vector r.
-   */
-  virtual ForceAndEnergy calc_force_and_energy(Vec3<real> r) = 0;
-};
-
-struct LennardJones : PairPotential {
-  real m_epsilon;
-  real m_sigma;
-
-  LennardJones(real sigma, real epsilon, real rcutoff)
-      : PairPotential(rcutoff), m_epsilon(epsilon), m_sigma(sigma) {};
-
-  ForceAndEnergy calc_force_and_energy(Vec3<real> r);
-
-  ~LennardJones() {};
-};
-
-#endif