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def test_acn():
"""Test ACN forces."""
from ase import Atoms
from ase.calculators.acn import ACN, m_me, r_mec, r_cn
dimer = Atoms('CCNCCN',
[(-r_mec, 0, 0),
(0, 0, 0),
(r_cn, 0, 0),
(r_mec, 3.7, 0),
(0, 3.7, 0),
(-r_cn, 3.7, 0)])
# Set mass of methyls
masses = dimer.get_masses()
masses[::3] = m_me
dimer.set_masses(masses)
dimer.calc = ACN(rc=5.0, width=2.0) # Put C-C distance in the cutoff range
F = dimer.get_forces()
print(F)
Fnum = dimer.calc.calculate_numerical_forces(dimer)
dF = Fnum - F
print(dF)
assert abs(dF).max() < 2e-6
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