def METHOD_NAME():
    assert openmc.data.atomic_mass('H1') == 1.007825031898
    assert openmc.data.atomic_mass('U235') == 235.043928117
    assert openmc.data.atomic_mass('Li6') == 6.01512288742
    assert openmc.data.atomic_mass('Pb220') == 220.025905
    with pytest.raises(KeyError):
        openmc.data.atomic_mass('U100')