Limited-Basis-Set Hartree-Fock Theory of NiF64−

Restricted self-consistent-field molecular-orbital calculations have been carried out for four states of the octahedral cluster Ni${\mathrm{F}}_6^{4-}$ in vacuo and with 38 point charges representing the perovskite lattice KNi${\mathrm{F}}_3$. For a contracted Gaussian-basis set with 15 more members than a minimal set, all integrals were computed. The value found for the ligand field splitting, 4670 ${\mathrm{cm}}^{-1\mathrm{}}$, is 35% smaller than the observed value. The Racah parameters do not reflect the expected decrease associated with cluster formation. The covalency estimates, $f_s\cong 0.18\%$ and $f_{\sigma }\cong 1.26\%$, are three times smaller than the observed values. Thus, as measured by the results, the calculation is neither significantly better nor significantly worse than previous calculations, which, however, include many ill-understood assumptions. In addition, our calculations indicate that omission of the Madelung potential from next-nearest neighbors is not a grievous approximation; that free-ion $d$ orbitals suffer only small distortions in the cluster; and that $p\pi$ bonding in states with partly open $t_{2g}$ orbitals is much smaller than $p\sigma$ bonding. This calculation is to be regarded as a precursor. An attempt is made to extract the implications so that selection of the least sensitive assumptions may be made for subsequent calculations.