docs: def zharkov_pel(v, temp, v0, e0, g, n, z, t_ref=300.,
                  three_r=3. * constants.R):
      """
      calculate electronic contributions in pressure for the Zharkov equation
      the equation can be found in Sokolova and Dorogokupets 2013
  
      :param v: unit-cell volume in A^3
      :param temp: temperature in K
      :param v0: unit-cell volume in A^3 at 1 bar
      :param e0: parameter in K-1 for the Zharkov equation
      :param g: parameter for the Zharkov equation
      :param n: number of atoms in a formula unit
      :param z: number of formula unit in a unit cell
      :param t_ref: reference temperature, 300 K
      :param three_r: 3 times gas constant
      :return: electronic contribution in GPa
      """
      v_mol = vol_uc2mol(v, z)
      x = v / v0
  #    a = a0 * np.power(x, m)
  
      def f(t):
          return three_r * n / 2. * e0 * np.power(x, g) * np.power(t, 2.) * \
              g / v_mol * 1.e-9
      return f(temp) - f(t_ref)