Improved calculation of electronic excitation energies of transition-metal compounds by a correlation energy correction: Application to CrF63− and NiF64−

Correlation energy as an empirical correction is introduced into otherwise ab initio‐type calculations of the electronic structure of trnasition metal complexes. The strong field MO electronic states are expanded in a series of free‐ion LS terms and cluster correlation energies for the SΓ states are calculated from the coefficients of such expansions and atomic spectral data. Numerical results are presented for ${\mathrm{NiF}}_6^{\text{4−}}$ and ${\mathrm{CrF}}_6^{\text{3−}}$ . The effects of this correction on calculated spectra in these two cases are discussed in comparison with the limited ($d^n$ ) configuration interaction. In both clusters the calculated spectra are significantly improved: the mean deviations (in cm⁻¹) from all observed transitions are reduced from 3420 to 680 in ${\mathrm{NiF}}_6^{\text{4−}}$ and from 2980 to 450 in ${\mathrm{CrF}}_6^{\text{3−}}$ . An analysis of the influence of the electronic delocalization on the calculated spectra suggests that the charge‐transfer states might be not very significant in an extended CI treatment of the correlation energy problem in these clusters.