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