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

{\displaystyle |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.

{{{2}}}

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 ^[2] and Ref. ^[3] 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.