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