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def test_hookean_pbc():
from ase import Atoms
from ase.calculators.emt import EMT
from ase.constraints import Hookean
L = 8. # length of the cubic box
d = 2.3 # Au-Au distance
cell = [L] * 3
positions = [[(L - d/2) % L , L/2, L/2], [(L + d/2) % L, L/2, L/2]]
a = Atoms('AuAu', cell=cell, positions=positions, pbc=True)
a.calc = EMT()
e1 = a.get_potential_energy()
constraint = Hookean(a1=0, a2=1, rt=1.1*d, k=10.)
a.set_constraint(constraint)
e2 = a.get_potential_energy()
a.set_pbc([False, True, True])
e3 = a.get_potential_energy()
assert abs(e1 - e2) < 1e-8
assert not abs(e1 - e3) < 1e-8
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