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import pytest
import numpy as np
from ase.md import VelocityVerlet
from ase.build import bulk
from ase.units import kB
from ase.md.velocitydistribution import MaxwellBoltzmannDistribution
from ase.calculators.idealgas import IdealGas
def test_idealgas():
rng = np.random.RandomState(17)
atoms = bulk('Kr').repeat((10, 10, 10))
assert len(atoms) == 1000
atoms.center(vacuum=100)
atoms.calc = IdealGas()
natoms = len(atoms)
md_temp = 1000
MaxwellBoltzmannDistribution(atoms, temperature_K=md_temp, rng=rng)
print("Temperature: {} K".format(atoms.get_temperature()))
with VelocityVerlet(atoms, timestep=0.1) as md:
for i in range(5):
md.run(5)
stress = atoms.get_stress(include_ideal_gas=True)
stresses = atoms.get_stresses(include_ideal_gas=True)
assert stresses.mean(0) == pytest.approx(stress)
pressure = -stress[:3].sum() / 3
pV = pressure * atoms.cell.volume
NkT = natoms * kB * atoms.get_temperature()
print(f"pV = {pV} NkT = {NkT}")
assert pV == pytest.approx(NkT, abs=1e-6)
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