LEFG

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

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

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

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

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

The asymmetry parameter is defined as $\eta = {(V_{yy} - V_{xx})}/ V_{zz}$. For so-called "quadrupolar nuclei", i.e., nuclei with nuclear spin I>1/2, NMR experiments can access V_zz 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 V_zz values into the C_q often encountered in NMR literature, one has to specify the nuclear quadrupole moment by means of the QUAD_EFG-tag. The output of $C_q$ is in MHz. See references and Ref. for a compilation of nuclear quadrupole moments.

$C_q = \frac{e Q V_{zz}}{h}$

Suppose a solid contains Al, C, and Si, then the QUAD_EFG tag could read:

QUAD_EFG = 146.6 33.27 0.0

$^{27}\mathrm{Al}$ is the stable isotope of Al with a natural abundance of 100% and $Q = 146.6$. The stable isotopes $^{12}\mathrm{C}$ and $^{13}\mathrm{C}$ are not quadrupolar nuclei, however, the radioactive $^{11}\mathrm{C}$ is. It has $Q = 33.27$. For Si it is pointless to calculate a $C_q$ since all stable isotopes have $I \le 1/2$. No moments are known for the other isotopes.

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

Related tags and articles

QUAD_EFG

Examples that use this tag