A CASSCF/CASPT2 study of the low-lying excited states of Mn2(CO)10

High-level ab initio quantum chemical methods are used to assign the low energetic part of the gas phase UV spectrum of dimanganese decacarbonyl. It is characterized by two bands at 26 700 and 29 740 cm−1 corresponding to the a1E1 (3dπ→σ*Mn–Mn) and a1B2 (σMn–Mn→σ*Mn–Mn) transitions which are calculated at 26 370 and 27 460 cm−1, respectively. In obtaining these values, the CASPT2 method has been pushed to its limits using the level shift correction in order to reduce the intruder states problem. It is shown that this method cannot be used safely without careful analysis of the excitation energies as a function of the level shift values. Our results are compared with recent density functional calculations.