def test_aic():
    """Test Atomic Counter Ion calc forces."""
    import numpy as np
    from ase import Atoms
    from ase.calculators.counterions import AtomicCounterIon as ACI

    atoms = Atoms('2Na', positions=np.array([[0,0,0], [0,0,4]]))

    atoms.calc = ACI(1, 1.6642, 0.0001201186, rc=4.5) 
    f = atoms.get_forces()
    df = atoms.calc.calculate_numerical_forces(atoms, 1e-6) - f
    print(df)
    assert abs(df).max() < 2e-6