J. Ehmann et M. Fahnle, INFLUENCE OF THE ELECTRONIC CORE POLARIZATION ON THE ELECTRIC-FIELD GRADIENTS IN SOLIDS, Physical review. B, Condensed matter, 55(12), 1997, pp. 7478-7491
In conventional band-structure calculations it is normally assumed tha
t closed electronic shells have a spherically symmetric charge density
. As a consequence of this approximation, these core states give no co
ntribution to the electric-field gradient (efg) at the nuclear site. I
n the present paper two equivalent methods for the computation of the
actual contribution of closed electron shells to the efg are presented
. In the first method the potential of the nuclear quadrupole moment i
s considered as a perturbation for the core electrons, which causes a
polarization of the core states, i.e., a deviation of the core-ch,uge
density from spherical symmetry. In this case the core contribution to
the efg can be calculated with the help of the Sternheimer function g
amma(r). The second method considers the nonspherical parts of the eff
ective crystal potential near the nucleus under consideration as a per
turbation for the core electrons. These two methods yield identical re
sults for the interaction energy of the nuclear quadrupole moment with
the calculated efg. They are compared with alternative treatments of
energetically high-lying core states within the framework of the full-
potential linearized-augmented-plane-wave method (semicore calculation
s and use of local orbitals). As test cases we calculate the efg at th
e nearest-neighbor sites of a substitutional Ni (Fe) atom in Cu (Al) a
nd the efg at a regular lattice site in hexagonal Mg. Additionally, re
sults for the relaxed atomic positions, the efg, and the asymmetry par
ameters around a substitutional Pd (V) atom in Cu (Al) are presented.