def _compute_term_5(self, C, mag, R):
        """
        (a20 + a21.*M + a22.*M.*M + a23.*M.*M.*M).*(d(r).^4)
        """
        return (
            (C['a20'] + C['a21'] * mag + C['a22'] * np.power(mag, 2) +
             C['a23'] * np.power(mag, 3)) * np.power(R, 4)
        )
(a20 + a21.*M + a22.*M.*M + a23.*M.*M.*M).*(d(r).^4)