code: def haeberlen_values(self):
          
          pas=self.principal_axis_system
          sigma_iso=pas.trace() / 3
          sigmas=np.diag(pas)
          sigmas=sorted(sigmas, key=lambda x: np.abs(x - sigma_iso))
          sigma_yy, sigma_xx, sigma_zz=sigmas
          delta_sigma=sigma_zz - 0.5 * (sigma_xx + sigma_yy)
          zeta=sigma_zz - sigma_iso
          eta=(sigma_yy - sigma_xx) / zeta
          return self.HaeberlenNotation(sigma_iso, delta_sigma, zeta, eta)
docs: Returns: the Chemical shielding tensor in Haeberlen Notation