The lowest excited electronic states of the permanganate ion MnO4− are calculated using a hierarchy of coupled cluster response approaches, as well as time-dependent density functional theory. It is shown that while full linear response coupled cluster with singles and doubles (or higher) performs well, that permanganate represents a stern test for approximate coupled cluster response models, and that problems can be traced to very large orbital relaxation effects. TD-DFT is reasonably robust although errors around 0.6 eV are still observed. In order to further investigate the strong correlations prevalent in the electronic ground state large-scale RASSCF calculations were also performed. Again very large orbital relaxation in the correlated wavefunction is observed. Although the system can qualitatively be described by a single configuration, multi-reference diagnostic values show that care must be taken in this and similar metal complexes.
Almeida, N. M. S., Mckinlay, R. G., & Paterson, M. J. (2015). Excited electronic states of MnO4−: Challenges for wavefunction and density functional response theories. Chemical Physics, 446, 86-91. https://doi.org/10.1016/j.chemphys.2014.11.011