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# LEFG

LEFG = .TRUE. | .FALSE.
Default: LEFG = .FALSE.

Description: The LEFG Computes the Electric Field Gradient at positions of the atomic nuclei.

For LEFG=.TRUE., the electric field gradient tensors at the positions of the atomic nuclei are calculated using the method of Petrilli et al.[1]

The EFG tensors are symmetric. The principal components Vii and asymmetry parameter η are printed for each atom. Following convention the principal components Vii are ordered such that:

${\displaystyle |V_{{zz}}|>|V_{{xx}}|>|V_{{yy}}|.}$

The asymmetry parameter is defined as η=(Vyy-Vxx)/Vzz. For so-called "quadrupolar nuclei", i.e., nuclei with nuclear spin I>1/2, NMR experiments can access Vzz and η.

Beware: Attaining convergence can require somewhat smaller EDIFF than the default of 1.e-4 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 *_pv or *_sv POTCARs) as well as explicit inclusion of augmentation channel(s) with d-projectors.

To convert the Vzz values into the Cq often encountered in NMR literature, one has to specify the nuclear quadrupole moment by means of the QUAD_EFG-tag.

Beware: for heavy nuclei inaccuracies are to be expected because of an incomplete treatement of relativistic effects.