def epsilon_to_kappa(r_k, epsilon, delta=0.16):
    """Convert frequency r_k and strain epsilon
    to corresponding r_k and kappa as consumed by
    functions in `latticegeneration`.

    Returns
    -------
    r_k2 : float
    kappa : float

    See also
    --------
    latticegeneration.generate_ks
    """
    return r_k / (1 - delta*epsilon), (1+epsilon) / (1 - delta*epsilon)