calculate thermal pressure

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

def cal_pth(self, v, temp):
        """
        calculate thermal pressure

        :param v: unit-cell volume in A^3
        :param temp: temperature in K
        :return: thermal pressure in GPa
        """
        params_t = self._set_params(self.params_therm)
        return constq_pth(v, temp, *params_t, self.n, self.z,
                          t_ref=self.t_ref, three_r=self.three_r)