APPLICATIONS OF LEVEL SHIFT CORRECTED PERTURBATION-THEORY IN ELECTRONIC SPECTROSCOPY

Multiconfigurational second-order perturbation theory (CASPT2) with a level shift technique used to reduce the effect of intruder states has been tested for applications in electronic spectroscopy. The followin g molecules have been studied: formamide, adenine, stilbene, Ni(CO)(4) , and a model compound for the active site in the blue copper protein plastocyanin, Cu(Im)(2)(SH)(SH2)(+). The results show that the level s hift technique can be used to remove the effects of the intruder state s in all these molecules. In some cases a drift in the energies as a f unction of the level shift is observed, which however is small enough that the normal error bar for CAsPT2 excitation energies (approximate to 0.3 eV) still holds.