def METHOD_NAME(self, n_typed_ar_system):
    top = n_typed_ar_system(n_sites=1)
    next(iter(top.atom_types)).set_expression("sigma + epsilon*r")
    with pytest.raises(EngineIncompatibilityError):
        top.save("out.mcf")
    alternate_lj = "4*epsilon*sigma**12/r**12 - 4*epsilon*sigma**6/r**6"
    next(iter(top.atom_types)).set_expression(alternate_lj)
    top.save("ar.mcf")