Charge transfer states. CASPT2/CASSCF

The choice of the active orbital space for the CASSCF calculations is a crucial step, and has turned out to be especially difficult in these proteins and other systems containing a Cu-thiolate bond. From earlier studies it was known that in complexes with first-row transition metal ions with many 3d electrons, the active space should include one correlating orbital for each of the doubly occupied 3d orbitals [28]. Therefore the starting active space contains 10 orbitals 3d and 3d"). In addition, it is necessary to add the 3p orbitals on Scys to describe correctly the covalent character of the Cu-Scys bond and also 2p and 3p orbitals on nitrogen and sulphur to describe charge-transfer states. The final active space therefore contain 11 or 12 active orbitals (12 active orbitals are at present the upper limit for the CASPT2 method). 

The preceding examples have indicated that the calculation of charge-transfer states in transition metal coordination compounds is certainly far from straightforward. However, a positive message is that, provided appropriate choices are made to keep the size of the CASSCF active space within limits, the subsequent CASPT2 are still accurate enough (with errors of at most 0.5 eV... 

For the CASPT2 calculations of the spectra, the active space was extended with the second Scys 3p orbital. No symmetry was used in the calculations, and all states were included in one state-averaged CASSCF calculation. With 13 electrons in 12 orbitals, seven states, consisting of all ligand-field states as well as the two Scys -> Cu charge-transfer states, could be included in the calculations. 

As for Cr(CO)g, a calculation of the complete charge transfer spectrum of Ni(CO)4, including singlet excited states of all possible symmetries, has been performed in a recent CASSCF/CASPT2 study [177]. Here the results obtained for the symmetry-allowed... 

Fig. 5 Photochemical paths studied with the CASSCF/CASPT2 method in the AT base pair to describe the photostability and tautomerism phenomena in the system (top) and energy profile for three different photoehemieal mechanisms of the base pair (bottom) concerted double proton transfer (1), stepwise double proton transfer via locally excited (LE) states (2), and stepwise double hydrogen transfer via charge transfer (CT) excited states (3).

---