Overriden to account for the fact the fit with volume**(2/3) instead
        of volume.

def _set_params(self):
        """
        Overriden to account for the fact the fit with volume**(2/3) instead
        of volume.
        """
        deriv0 = np.poly1d(self.eos_params)
        deriv1 = np.polyder(deriv0, 1)
        deriv2 = np.polyder(deriv1, 1)
        deriv3 = np.polyder(deriv2, 1)

        for x in np.roots(deriv1):
            if x > 0 and deriv2(x) > 0:
                v0 = x**(-3./2.)
                break
        else:
            raise EOSError("No minimum could be found")

        derivV2 = 4./9. * x**5. * deriv2(x)
        derivV3 = (-20./9. * x**(13./2.) * deriv2(x) - 8./27. *
                   x**(15./2.) * deriv3(x))
        b0 = derivV2 / x**(3./2.)
        b1 = -1 - x**(-3./2.) * derivV3 / derivV2

        # e0, b0, b1, v0
        self._params = [deriv0(v0**(-2./3.)), b0, b1, v0]