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# fmt: off
import numpy as np
from ase import units
from ase.build import bulk
from ase.calculators.emt import EMT
from ase.md.langevin import Langevin
from ase.md.velocitydistribution import MaxwellBoltzmannDistribution, Stationary
def test_langevin_com():
"""Check that the center of mass does not move during Langevin dynamics.
In particular, test that this does not happen with atoms with different
mass present (issue #1044).
"""
# parameters
size = 2
T = 300
dt = 0.01
# setup
atoms = bulk('CuAg', 'rocksalt', a=4.0).repeat(size)
atoms.pbc = False
atoms.calc = EMT()
MaxwellBoltzmannDistribution(atoms, temperature_K=T)
Stationary(atoms)
mtot = atoms.get_momenta().sum(axis=0)
print('initial momenta', mtot)
print('initial forces', atoms.get_forces().sum(axis=0))
# run NVT
with Langevin(atoms, dt * units.fs, temperature_K=T, friction=0.02) as dyn:
dyn.run(10)
m2 = atoms.get_momenta().sum(axis=0)
print('momenta', m2)
print('forces', atoms.get_forces().sum(axis=0))
print()
assert np.linalg.norm(m2) < 1e-8
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