The electronic spectroscopy of transition metal carbonyls: The tough case of Cr(CO)6

The electronic spectroscopy of Cr(CO)6, representative molecule for transition metal carbonyls is revisited by various quantum chemical methods from self consistent size consistent single double configuration interaction, equation of motion coupled cluster single double, multi state complete active space second-order perturbation, MS-CASPT2, and time-dependent density functional theory (TD-DFT). Comparison is done with previous CASPT2 and TD-DFT studies. Compared to experiment, most methods (except MS-CASPT2) do not give completely satisfactory quantitative results, which emphasizes the difficulty to perform highly accurate spectroscopy calculations. However, the results allow to clearly assign the nature of the different states, compared to experiment.