code: def constq_grun(v, v0, gamma0, q):
      
      x = v / v0
      return gamma0 * np.power(x, q)
docs: calculate Gruneisen parameter for constant q
  
      :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 q: logarithmic derivative of Grunseinen parameter
      :return: Gruneisen parameter at a given volume