calculate pressure from anharmonic contributions

        :param v: unit-cell volume in A^3
        :param temp: temperature in K
        :return: pressure in GPa

def cal_panh(self, v, temp):
        """
        calculate pressure from anharmonic contributions

        :param v: unit-cell volume in A^3
        :param temp: temperature in K
        :return: pressure in GPa
        """
        if (self.eqn_anh is None) or (self.params_anh is None):
            return np.zeros_like(v)
        params = self._set_params(self.params_anh)
        return func_anh[self.eqn_anh](v, temp, *params,
                                      self.n, self.z, t_ref=self.t_ref,
                                      three_r=self.three_r)