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import numpy as np
import pytest
from ase.build import bulk
from ase import units
from ase.md.velocitydistribution import MaxwellBoltzmannDistribution
from ase.calculators.harmonic import SpringCalculator
from ase.md.switch_langevin import SwitchLangevin
@pytest.mark.slow
def test_langevin_switching():
# params
size = 6
T = 300
n_steps = 500
k1 = 2.0
k2 = 4.0
dt = 10
# for reproducibility
np.random.seed(42)
# setup atoms and calculators
atoms = bulk('Al').repeat(size)
calc1 = SpringCalculator(atoms.positions, k1)
calc2 = SpringCalculator(atoms.positions, k2)
# theoretical diff
n_atoms = len(atoms)
calc1.atoms = atoms
calc2.atoms = atoms
F1 = calc1.get_free_energy(T) / n_atoms
F2 = calc2.get_free_energy(T) / n_atoms
dF_theory = F2 - F1
# switch_forward
dyn_forward = SwitchLangevin(atoms, calc1, calc2, dt * units.fs,
temperature_K=T, friction=0.01,
n_eq=n_steps, n_switch=n_steps)
MaxwellBoltzmannDistribution(atoms, temperature_K=2 * T)
dyn_forward.run()
dF_forward = dyn_forward.get_free_energy_difference() / len(atoms)
# switch_backwards
dyn_backward = SwitchLangevin(atoms, calc2, calc1, dt * units.fs,
temperature_K=T, friction=0.01,
n_eq=n_steps, n_switch=n_steps)
MaxwellBoltzmannDistribution(atoms, temperature_K=2 * T)
dyn_backward.run()
dF_backward = -dyn_backward.get_free_energy_difference() / len(atoms)
# summary
dF_switch = (dF_forward + dF_backward) / 2.0
error = dF_switch - dF_theory
# print('delta_F analytical: {:12.6f} eV/atom'.format(dF_theory))
# print('delta_F forward: {:12.6f} eV/atom'.format(dF_forward))
# print('delta_F backward: {:12.6f} eV/atom'.format(dF_backward))
# print('delta_F average: {:12.6f} eV/atom'.format(dF_switch))
# print('delta_F error: {:12.6f} eV/atom'.format(error))
assert abs(error) < 1e-3
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