An EFG tensor can be transformed into its principal axis system (PAS) V PAS , shown in Fig. , by diagonalization at a given nuclear position. This yields the eigenvalues V XX , V YY , and In the PAS, V ZZ describes the magnitude of the tensor, while V YY and V XX describe its width. Major standard NMR observables like the quadrupolar coupling constant C Q and the quadrupolar asymmetry parameter η directly derive from these eigenvalues. C Q defines the coupling strength between the EFG and the applied magnetic field B 0