Compatibility function, return mode=metal, when
    available, mode=hill, when not (ASE <= 3.13)

def get_chemical_formula(atoms, mode='metal'):
    """
    Compatibility function, return mode=metal, when
    available, mode=hill, when not (ASE <= 3.13)
    """
    try:
        return atoms.get_chemical_formula(mode=mode)
    except ValueError:
        return atoms.get_chemical_formula(mode='hill')