Overriden since this eos works with volume**(2/3) instead of volume.

def fit(self, order=3):
        """
        Overriden since this eos works with volume**(2/3) instead of volume.
        """
        x = self.volumes**(-2./3.)
        self.eos_params = np.polyfit(x, self.energies, order)
        self._set_params()