From Intermetallic compounds: Principles and Practice, Vol. I:
        Principles Chapter 9 pages 195-210 by M. Mehl. B. Klein,
        D. Papaconstantopoulos.
        case where n=0

def _func(self, volume, params):
        """
        From Intermetallic compounds: Principles and Practice, Vol. I:
        Principles Chapter 9 pages 195-210 by M. Mehl. B. Klein,
        D. Papaconstantopoulos.
        case where n=0
        """
        e0, b0, b1, v0 = tuple(params)
        return (e0
                + 9.0 / 8.0 * b0 * v0 * ((v0 / volume)**(2.0/3.0) - 1.0) ** 2
                + 9.0 / 16.0 * b0 * v0 * (b1 - 4.) *
                ((v0 / volume)**(2.0/3.0) - 1.0) ** 3)