The following flags control the behaviour of VASP:
LEFG = .TRUE. | .FALSE.
LEFGswitches on the calculation of the electric field gradient tensors. The EFG tensors are symmetric. The principal components and asymmetry parameter are printed for each atom. Following convention the principal components are ordered such that:
Beware: Attaining convergence can require somewhat smaller
EDIFF than the default of
and somewhat larger cutoff
ENCUT than default with
PREC=A. Moreover, the calculation of
EFGs typically requires high quality PAW data sets. Semi-core electrons can be important (check with
*_sv POTCARs) as well as explicit inclusion of augmentation channel(s) with -projectors.
QUAD_EFG-tag (default: 1.) This tag allows the conversion by VASP of the values into the often encountered in NMR literature. The conversion formula ( is the element and isotope specific quadrupole moment):
QUAD_EFG-tag consists of the nuclear quadrupole moment in millibarns for each atomic species, in the same order as in the POTCAR file. The output is in MHz. See Ref.  for a compilation of nuclear quadrupole moments.
Suppose a solid contains Al, C and Si, than the
QUAD_EFG-tag could read:
QUAD_EFG = 146.6 33.27 0Al is the stable isotope of Al with a natural abundance of 100 % and . The stable isotopes C and C are not quadrupolar nuclei, however, the radioactive C is. It has . For Si it is pointless to calculate a : Again all stable isotopes have . No moments are known for the other isotopes.
Beware: several definitions of are used in the NMR community.
Beware: for heavy nuclei inaccuracies are to be expected because of an incomplete treatement of relativistic effects.