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. .

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.

Mind: 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.

Recommendations and advice

Tight settings are required for calculating the electric field gradient.

Input parameters

• A larger ENCUT value than usual, generally much higher than the value given by ENMAX in the POTCAR file, e.g. 800 eV for C in diamond, rather than the standard 400 eV.
• A small EDIFF is required to provide converged chemical shifts, e.g. 1E-8 eV.
• Tighter precision, e.g. PREC = Accurate.
• Non-spherical contributions to the gradient of the density inside PAW spheres, i.e. LASPH = .TRUE.

Structure

• The structure is extremely important, so using the experimental structure can improve results.

PAW pseudopotentials

• The use of PAW potentials has a strong influence, GW POTCAR files often improve values.
• Semi-core electrons can be important (check the POSCAR files with *_pv or *_sv) as well as explicit inclusion of augmentation channels with $d$-projectors.

Mind: Several definitions of $C_q$ are used in the NMR community, ensure that you are comparing between the same definitions in calculation and experiment.

Important: 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