docs: def zharkov_panh(v, temp, v0, a0, m, n, z, t_ref=300.,
                   three_r=3. * constants.R):
      """
      calculate pressure from anharmonicity for Zharkov equation
      the equation is from Dorogokupets 2015
  
      :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 a0: parameter in K-1 for the Zharkov equation
      :param m: parameter for the Zharkov equation
      :param n: number of elements in a chemical formula
      :param z: number of formula unit in a unit cell
      :param three_r: 3 times gas constant
      :return: anharmonic contribution for pressure 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. * a * m / v_mol * np.power(t, 2.) * 1.e-9
  
      return f(temp) - f(t_ref)