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
        """
        if (self.eqn_th is None) or (self.params_th is None):
            return np.zeros_like(v)
        params = self._set_params(self.params_th)
        return func_th[self.eqn_th](v, temp, *params,
                                    self.n, self.z, t_ref=self.t_ref,
                                    three_r=self.three_r)
calculate thermal pressure

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