Returns a chemical shielding tensor aligned to the principle axis system
        so that only the 3 diagnol components are non-zero

def principal_axis_system(self):
        """
        Returns a chemical shielding tensor aligned to the principle axis system
        so that only the 3 diagnol components are non-zero
        """
        return ChemicalShielding(np.diag(np.sort(np.linalg.eigvals(self))))