code: def mehring_values(self):
          
          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)
docs: Returns: the Chemical shielding tensor in Mehring Notation