Electric-field-gradient calculations for systems with large extended-core-state contributions

Electric-field-gradient (EFG) calculations for ${\mathrm{TiO}}_2$ in the rutile structure using the standard full-potential linearized-augmented-plane-wave (LAPW) method have shown that the contribution of the Ti 3p semicore states is comparable to that of the valence electrons in contrast to other systems studied so far, where the latter dominate. This makes the treatment of the 3p and 4p states of Ti an important issue. Two-energy-window calculations with different Ti p energy linearization parameters or different sphere radii yield drastically different calculated EFG’s. The results of standard LAPW calculations are in poor agreement with experiment. An extension of the LAPW basis with local orbitals, however, yields EFG results close to experiment and removes the sensitivity to sphere radii and energy parameters. The large contribution from the semicore states remains and thus must be considered to be real.