TestReferenceDrudeForce.cpp

/* -------------------------------------------------------------------------- *
 *                                   OpenMM                                   *
 * -------------------------------------------------------------------------- *
 * This is part of the OpenMM molecular simulation toolkit originating from   *
 * Simbios, the NIH National Center for Physics-Based Simulation of           *
 * Biological Structures at Stanford, funded under the NIH Roadmap for        *
 * Medical Research, grant U54 GM072970. See https://simtk.org.               *
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 * Portions copyright (c) 2013 Stanford University and the Authors.           *
 * Authors: Peter Eastman                                                     *
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/**
 * This tests the Reference implementation of DrudeForce.
 */

#include "openmm/internal/AssertionUtilities.h"
#include "openmm/Context.h"
#include "openmm/NonbondedForce.h"
#include "openmm/Platform.h"
#include "openmm/System.h"
#include "openmm/VerletIntegrator.h"
#include "openmm/DrudeForce.h"
#include "SimTKUtilities/SimTKOpenMMUtilities.h"
#include <iostream>
#include <vector>

using namespace OpenMM;
using namespace std;

void validateForce(System& system, vector<Vec3>& positions, double expectedEnergy) {
    // Given a System containing a Drude force, check that its energy has the expected value.
    
    VerletIntegrator integ(1.0);
    Platform& platform = Platform::getPlatformByName("Reference");
    Context context(system, integ, platform);
    context.setPositions(positions);
    State state = context.getState(State::Energy | State::Forces);
    ASSERT_EQUAL_TOL(expectedEnergy, state.getPotentialEnergy(), 1e-5);
    
    // Try moving each particle along each axis, and see if the energy changes by the correct amount.
    
    double offset = 1e-3;
    for (int i = 0; i < system.getNumParticles(); i++)
        for (int j = 0; j < 3; j++) {
            vector<Vec3> offsetPos = positions;
            offsetPos[i][j] = positions[i][j]-offset;
            context.setPositions(offsetPos);
            double e1 = context.getState(State::Energy | State::Forces).getPotentialEnergy();
            offsetPos[i][j] = positions[i][j]+offset;
            context.setPositions(offsetPos);
            double e2 = context.getState(State::Energy | State::Forces).getPotentialEnergy();
            ASSERT_EQUAL_TOL(state.getForces()[i][j], (e1-e2)/(2*offset), 1e-3);
        }
}

void testSingleParticle() {
    const double k = 1.5;
    const double charge = 0.1;
    const double alpha = charge*charge/k;
    System system;
    system.addParticle(1.0);
    system.addParticle(1.0);
    DrudeForce* drude = new DrudeForce();
    drude->addParticle(1, 0, -1, -1, -1, charge, alpha, 1, 1);
    system.addForce(drude);
    vector<Vec3> positions(2);
    positions[0] = Vec3(-1, 0, 0);
    positions[1] = Vec3(2, 0, 0);
    validateForce(system, positions, 0.5*k*3*3);
}

void testAnisotropicParticle() {
    const double k = 1.5;
    const double charge = 0.1;
    const double alpha = charge*charge/k;
    const double a1 = 0.8;
    const double a2 = 1.1;
    const double k1 = k/a1;
    const double k2 = k/a2;
    const double k3 = k/(3-a1-a2);
    System system;
    system.addParticle(1.0);
    system.addParticle(1.0);
    system.addParticle(1.0);
    system.addParticle(1.0);
    system.addParticle(1.0);
    DrudeForce* drude = new DrudeForce();
    drude->addParticle(1, 0, 2, 3, 4, charge, alpha, a1, a2);
    system.addForce(drude);
    vector<Vec3> positions(5);
    positions[0] = Vec3(0, 0, 0);
    positions[1] = Vec3(0.1, -0.5, 0.8);
    positions[2] = Vec3(0, 2, 0);
    positions[3] = Vec3(1, 2, 0);
    positions[4] = Vec3(1, 2, 3);
    validateForce(system, positions, 0.5*k1*0.5*0.5 + 0.5*k2*0.8*0.8 + 0.5*k3*0.1*0.1);
}

double computeScreening(double r, double thole, double alpha1, double alpha2) {
    double u = r*thole/pow(alpha1*alpha2, 1.0/6.0);
    return 1.0-(1.0+u/2)*exp(-u);
}

void testThole() {
    const double k = 1.5;
    const double charge = 0.1;
    const double alpha = charge*charge/k;
    const double thole = 2.5;
    System system;
    system.addParticle(1.0);
    system.addParticle(1.0);
    system.addParticle(1.0);
    system.addParticle(1.0);
    DrudeForce* drude = new DrudeForce();
    drude->addParticle(1, 0, -1, -1, -1, charge, alpha, 1, 1);
    drude->addParticle(3, 2, -1, -1, -1, charge, alpha, 1, 1);
    drude->addScreenedPair(0, 1, thole);
    system.addForce(drude);
    vector<Vec3> positions(4);
    positions[0] = Vec3(0, 0, 0);
    positions[1] = Vec3(0, -0.5, 0);
    positions[2] = Vec3(1, 0, 0);
    positions[3] = Vec3(1, 0, 0.3);
    double energySpring1 = 0.5*k*0.5*0.5;
    double energySpring2 = 0.5*k*0.3*0.3;
    double energyDipole = 0.0;
    double q[] = {-charge, charge, -charge, charge};
    for (int i = 0; i < 2; i++)
        for (int j = 2; j < 4; j++) {
            Vec3 delta = positions[i]-positions[j];
            double r = sqrt(delta.dot(delta));
            energyDipole += ONE_4PI_EPS0*q[i]*q[j]*computeScreening(r, thole, alpha, alpha)/r;
        }
    validateForce(system, positions, energySpring1+energySpring2+energyDipole);
}

int main() {
    try {
        testSingleParticle();
        testAnisotropicParticle();
        testThole();
    }
    catch(const std::exception& e) {
        std::cout << "exception: " << e.what() << std::endl;
        std::cout << "FAIL - ERROR.  Test failed." << std::endl;
        return 1;
    }
    std::cout << "Done" << std::endl;
    return 0;
}