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def test_tip4p():
"""Test TIP4P forces."""
from math import cos, sin
from ase import Atoms
from ase.calculators.tip4p import TIP4P, rOH, angleHOH
r = rOH
a = angleHOH
dimer = Atoms('H2OH2O',
[(r * cos(a), 0, r * sin(a)),
(r, 0, 0),
(0, 0, 0),
(r * cos(a / 2), r * sin(a / 2), 0),
(r * cos(a / 2), -r * sin(a / 2), 0),
(0, 0, 0)])
# tip4p sequence OHH, OHH, ..
dimer = dimer[[2]]+dimer[:2]+dimer[[-1]]+dimer[3:5]
dimer.positions[3:, 0] += 2.8
dimer.calc = TIP4P(rc=4.0, width=2.0) # put O-O distance in the cutoff range
F = dimer.get_forces()
print(F)
dF = dimer.calc.calculate_numerical_forces(dimer) - F
print(dF)
assert abs(dF).max() < 2e-6
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