docs: def cal_pel(self, v, temp):
          """
          calculate pressure from electronic contributions
  
          :param v: unit-cell volume in A^3
          :param temp: temperature in K
          :return: pressure in GPa
          """
          if (self.eqn_el is None) or (self.params_el is None):
              return np.zeros_like(v)
          params = self._set_params(self.params_el)
          return func_el[self.eqn_el](v, temp, *params,
                                      self.n, self.z, t_ref=self.t_ref,
                                      three_r=self.three_r)