code: def cal_p_kunc(v, k, order=5, uncertainties=True):
      
      v0 = k[0]
      k0 = k[1]
      k0p = k[2]
      x = np.power(v / v0, 1. / 3.)
      f1 = (1. - x) / (np.power(x, order))
      if uncertainties:
          f2 = unp.exp((1.5 * k0p - order + 0.5) * (1. - x))
      else:
          f2 = np.exp((1.5 * k0p - order + 0.5) * (1. - x))
      p = 3. * k0 * f1 * f2
      return p
docs: calculate Kunc EOS,
      see Dorogokupets2015 for functional form
  
      :param v: unit-cell volume in A^3
      :param k: [v0, k0, k0p]
      :param order: order for the Kunc equation
      :param uncertainties: use of uncertainties package
      :return: pressure in GPa
      :note: internal function