def _func(self, volume, params):
        """
        Pourier-Tarantola equation from PRB 70, 224107
        """
        e0, b0, b1, v0 = tuple(params)
        eta = (volume / v0) ** (1. / 3.)
        squiggle = -3.*np.log(eta)
        return e0 + b0 * v0 * squiggle ** 2 / 6. * (3. + squiggle * (b1 - 2))