Returns: the Chemical shielding tensor in Haeberlen Notation

def haeberlen_values(self):
        """
        Returns: the Chemical shielding tensor in Haeberlen Notation
        """
        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)