void testIdealGas() {
const int numParticles = 64;
const int frequency = 10;
const int steps = 1000;
const double pressure = 1.5;
const double pressureInMD = pressure*(AVOGADRO*1e-25); // pressure in kJ/mol/nm^3
const double temp[] = {300.0, 600.0, 1000.0};
const double initialVolume = numParticles*BOLTZ*temp[1]/pressureInMD;
const double initialLength = std::pow(initialVolume, 1.0/3.0);
// Create a gas of noninteracting particles.
ReferencePlatform platform;
System system;
system.setDefaultPeriodicBoxVectors(Vec3(initialLength, 0, 0), Vec3(0, 0.5*initialLength, 0), Vec3(0, 0, 2*initialLength));
vector<Vec3> positions(numParticles);
OpenMM_SFMT::SFMT sfmt;
init_gen_rand(0, sfmt);
for (int i = 0; i < numParticles; ++i) {
system.addParticle(1.0);
positions[i] = Vec3(initialLength*genrand_real2(sfmt), 0.5*initialLength*genrand_real2(sfmt), 2*initialLength*genrand_real2(sfmt));
}
MonteCarloBarostat* barostat = new MonteCarloBarostat(pressure, temp[0], frequency);
system.addForce(barostat);
ASSERT(barostat->usesPeriodicBoundaryConditions());
ASSERT(system.usesPeriodicBoundaryConditions());
// Test it for three different temperatures.
for (int i = 0; i < 3; i++) {
barostat->setTemperature(temp[i]);
LangevinIntegrator integrator(temp[i], 0.1, 0.01);
Context context(system, integrator, platform);
context.setPositions(positions);
// Let it equilibrate.
integrator.step(10000);
// Now run it for a while and see if the volume is correct.
double volume = 0.0;
for (int j = 0; j < steps; ++j) {
Vec3 box[3];
context.getState(0).getPeriodicBoxVectors(box[0], box[1], box[2]);
volume += box[0][0]*box[1][1]*box[2][2];
ASSERT_EQUAL_TOL(0.5*box[0][0], box[1][1], 1e-5);
ASSERT_EQUAL_TOL(2*box[0][0], box[2][2], 1e-5);
integrator.step(frequency);
}
volume /= steps;
double expected = (numParticles+1)*BOLTZ*temp[i]/pressureInMD;
ASSERT_USUALLY_EQUAL_TOL(expected, volume, 3/std::sqrt((double) steps));
}
}
void testRandomSeed() {
const int numParticles = 8;
const double temp = 100.0;
const double pressure = 1.5;
ReferencePlatform platform;
System system;
system.setDefaultPeriodicBoxVectors(Vec3(8, 0, 0), Vec3(0, 8, 0), Vec3(0, 0, 8));
VerletIntegrator integrator(0.01);
NonbondedForce* forceField = new NonbondedForce();
forceField->setNonbondedMethod(NonbondedForce::CutoffPeriodic);
for (int i = 0; i < numParticles; ++i) {
system.addParticle(2.0);
forceField->addParticle((i%2 == 0 ? 1.0 : -1.0), 1.0, 5.0);
}
system.addForce(forceField);
MonteCarloBarostat* barostat = new MonteCarloBarostat(pressure, temp, 1);
system.addForce(barostat);
ASSERT(barostat->usesPeriodicBoundaryConditions());
ASSERT(system.usesPeriodicBoundaryConditions());
vector<Vec3> positions(numParticles);
vector<Vec3> velocities(numParticles);
for (int i = 0; i < numParticles; ++i) {
positions[i] = Vec3((i%2 == 0 ? 2 : -2), (i%4 < 2 ? 2 : -2), (i < 4 ? 2 : -2));
velocities[i] = Vec3(0, 0, 0);
}
// Try twice with the same random seed.
barostat->setRandomNumberSeed(5);
Context context(system, integrator, platform);
context.setPositions(positions);
context.setVelocities(velocities);
integrator.step(10);
State state1 = context.getState(State::Positions);
context.reinitialize();
context.setPositions(positions);
context.setVelocities(velocities);
integrator.step(10);
State state2 = context.getState(State::Positions);
// Try twice with a different random seed.
barostat->setRandomNumberSeed(10);
context.reinitialize();
context.setPositions(positions);
context.setVelocities(velocities);
integrator.step(10);
State state3 = context.getState(State::Positions);
context.reinitialize();
context.setPositions(positions);
context.setVelocities(velocities);
integrator.step(10);
State state4 = context.getState(State::Positions);
// Compare the results.
for (int i = 0; i < numParticles; i++) {
for (int j = 0; j < 3; j++) {
ASSERT(state1.getPositions()[i][j] == state2.getPositions()[i][j]);
ASSERT(state3.getPositions()[i][j] == state4.getPositions()[i][j]);
ASSERT(state1.getPositions()[i][j] != state3.getPositions()[i][j]);
}
}
}
