Dulk (1985) equations 7 and 20, assuming pure hydrogen.

def calc_freefree_eta(ne, t, hz):
    """Dulk (1985) equations 7 and 20, assuming pure hydrogen."""
    kappa = calc_freefree_kappa(ne, t, hz)
    return kappa * cgs.k * t * hz**2 / cgs.c**2