code: def tange_grun(v, v0, gamma0, a, b):
      
      x = v / v0
      return gamma0 * (1. + a * (np.power(x, b) - 1.))
docs: calculate Gruneisen parameter for the Tange equation
  
      :param v: unit-cell volume in A^3
      :param v0: unit-cell volume in A^3 at 1 bar
      :param gamma0: Gruneisen parameter at 1 bar
      :param a: volume-independent adjustable parameters
      :param b: volume-independent adjustable parameters
      :return: Gruneisen parameter