Returns: the Chemical shielding tensor in Mehring Notation

def mehring_values(self):
        """
        Returns: the Chemical shielding tensor in Mehring Notation
        """
        pas=self.principal_axis_system
        sigma_iso=pas.trace() / 3
        sigma_11, sigma_22, sigma_33=np.diag(pas)
        return self.MehringNotation(sigma_iso, sigma_11, sigma_22, sigma_33)