docs: 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)