Ab initio model potential embedded cluster calculation of the absorption spectrum of Cs2GeF6:Mn4+. Large discrepancies between theory and experiment

Abstract

Ab initio model potential (AIMP) embedded cluster calculations of the absorption spectrum of Cs2GeF6:Mn4+ show unusually large dicrepancies with very accurate experimental data in electronic transition energies that involve an effective single excitation within the octahedral Mn(3d3) manifold: \documentclass{article}\pagestyle{empty}\begin{document}$t_{2g}^{3}\to t_{2g}^{2}e_{g}$\end{document}; good results are obtained only for lower, intraconfigurational excitations that can be approximately described as metal \documentclass{article}\pagestyle{empty}\begin{document}$t_{2g}^{3}\to t_{2g}^{3}$\end{document} transitions. The large discrepancies are in clear contrast with previous (good) results obtained for similar materials with the same methods; their origins have been investigated here. Peculiar to the Cs2GeF6:Mn4+ material, as compared to V2+ and Cr3+ isoelectronic impurity ions in similar fluoride hosts, are a shorter metal–ligand distance, larger crystal field, and higher oxidation state. Consequently, it has been considered that the usual, truncated configuration interaction treatments might be failing in describing the proper role of ligand-to-metal charge transfer and non-negligible F–F bonding interactions in the tight MnF62− cluster. Complete and restricted active space self-consistent-field (CASSCF, RASSCF), and averaged coupled pair aproximation (ACPF) calculations have been done in the embedded MnF62− cluster in an attempt to investigate these factors. The effects of the dn and charge transfer (CT) configuration interactions on the wave functions and energies of the ground and excited states are found to be very state dependent. In particular, the inclusion of different CT configurations in the multiconfigurational space leads to very large variations on the calculated transition energies. These large differential nondynamic interactions appear to be peculiar to this intermediate oxidation state transition metal system and suggests that dynamic correlation treatments based on larger multireferences than those that are nowadays computationally feasible are needed. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 623–635, 2000