def n_eff(n):
    """Return the effective principal quantum number

    Args:
        n : (int)
            Principal quantum number

    .. note::
       The values are taken from J. A. Pople, D. L. Beveridge,
       "Approximate Molecular Orbital Theory", McGraw-Hill, 1970
    """

    values = {1: 1.0, 2: 2.0, 3: 3.0, 4: 3.7, 5: 4.0, 6: 4.2}

    return values.get(n, None)