Dulk (1985) eq 20, assuming pure hydrogen.

def calc_freefree_kappa(ne, t, hz):
    """Dulk (1985) eq 20, assuming pure hydrogen."""
    return 9.78e-3 * ne**2 * hz**-2 * t**-1.5 * (24.5 + np.log(t) - np.log(hz))