TestReferenceRandom.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.               *
 *                                                                            *
 * Portions copyright (c) 2008 Stanford University and the Authors.           *
 * Authors: Peter Eastman                                                     *
 * Contributors:                                                              *
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 * The above copyright notice and this permission notice shall be included in *
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/**
 * This tests the reference implementation of random number generation.
 */

#include "openmm/internal/AssertionUtilities.h"
#include "SimTKOpenMMUtilities.h"
#include "openmm/Context.h"
#include "ReferencePlatform.h"
#include "openmm/VerletIntegrator.h"
#include <iostream>

using namespace OpenMM;
using namespace std;

void testGaussian() {
    const int numValues = 10000000;
    double mean = 0.0;
    double var = 0.0;
    double skew = 0.0;
    double kurtosis = 0.0;
    for (int i = 0; i < numValues; i++) {
        double value = SimTKOpenMMUtilities::getNormallyDistributedRandomNumber();
        mean += value;
        var += value*value;
        skew += value*value*value;
        kurtosis += value*value*value*value;
    }
    mean /= numValues;
    var /= numValues;
    skew /= numValues;
    kurtosis /= numValues;
    double c2 = var-mean*mean;
    double c3 = skew-3*var*mean+2*mean*mean*mean;
    double c4 = kurtosis-4*skew*mean-3*var*var+12*var*mean*mean-6*mean*mean*mean*mean;
    ASSERT_EQUAL_TOL(0.0, mean, 0.01);
    ASSERT_EQUAL_TOL(1.0, c2, 0.01);
    ASSERT_EQUAL_TOL(0.0, c3, 0.01);
    ASSERT_EQUAL_TOL(0.0, c4, 0.01);
}

void testRandomVelocities() {
    // Create a system.
    
    const int numParticles = 10000;
    const double temperture = 100.0;
    ReferencePlatform platform;
    System system;
    VerletIntegrator integrator(0.01);
    for (int i = 0; i < numParticles; ++i)
        system.addParticle(10.0+sin(0.1*i));
    for (int i = 0; i < numParticles-1; ++i)
        system.addConstraint(i, i+1, 1.0);
    Context context(system, integrator, platform);
    vector<Vec3> positions(numParticles);
    for (int i = 0; i < numParticles; ++i)
        positions[i] = Vec3(i/2, (i+1)/2, 0);
    context.setPositions(positions);
    
    // Ask the context to generate random velocities.
    
    context.setVelocitiesToTemperature(temperture);
    State state = context.getState(State::Velocities);
    
    // See if they respect constraints.
    
    for (int i = 1; i < numParticles; i++) {
        Vec3 v1 = state.getVelocities()[i-1];
        Vec3 v2 = state.getVelocities()[i];
        double vel = (v1-v2).dot(positions[i-1]-positions[i]);
        ASSERT_EQUAL_TOL(0.0, vel, 2e-5);
    }
    
    // See if the temperature is correct.

    double ke = 0;
    for (int i = 0; i < numParticles; i++) {
        Vec3 v = state.getVelocities()[i];
        ke += 0.5*system.getParticleMass(i)*v.dot(v);
    }
    double expected = 0.5*system.getNumDegreesOfFreedom()*BOLTZ*temperture;
    ASSERT_USUALLY_EQUAL_TOL(expected, ke, 4/sqrt((double) numParticles));
}

int main() {
    try {
        testGaussian();
        testRandomVelocities();
    }
    catch(const exception& e) {
        cout << "exception: " << e.what() << endl;
        return 1;
    }
    cout << "Done" << endl;
    return 0;
}