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# fmt: off
import math
import pytest
from ase import Atoms
from ase.build import molecule
from ase.utils.ff import VdW, get_vdw_potential_value
@pytest.fixture()
def atoms() -> Atoms:
mol = molecule("H2")
mol.center(5.0)
return mol
def test_utils_ff_vdw(atoms: Atoms) -> None:
"""Check the equivalent van der Waals energies."""
e1, e2, s1, s2, scale = 0.646, 1.557, 0.777, 0.724, 1.2236
vdw_a = VdW(
0, 1, epsiloni=e1, epsilonj=e2, sigmai=s1, sigmaj=s2, scale=scale
)
vdw_p = VdW(
0, 1, epsilonij=math.sqrt(e1 * e2), sigmaij=(s1 + s2) / 2.0, scale=scale
)
e_ref = get_vdw_potential_value(atoms, vdw_a)
assert get_vdw_potential_value(atoms, vdw_p) == pytest.approx(e_ref)
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