| Abstract
| - CASSCF and CAS-PT2 calculations are performed for the ground and excited states of radical cations consistingof two and three nucleobases. The generalized Mulliken−Hush approach is employed for estimating electroniccouplings for hole transfer in the π-stacks. We compare the CASSCF results with data obtained withinKoopmans' approximation. The calculations show that an excess charge in the ground and excited states inthe systems is quite localized on a single base both at the CASSCF level and in Koopmans' picture. However,the CASSCF calculations point to a larger degree of localization and, in line with this, smaller transitiondipole moments. The agreement between the CAS-PT2 corrected energy gaps and the values estimated withKoopmans' theorem is better, with the CAS-PT2 calculations giving somewhat smaller gaps. Overall, bothfactors result in smaller CASSCF/CAS-PT2 couplings, which are reduced by up to 40% of the couplingscalculated using Koopmans' approximation. The tabulated data can be used as benchmark values for theelectronic couplings of stacked nucleobases. For the base trimers, comparison of the results obtained withintwo- and three-state models show that the multistate treatment should be applied to derive reliable estimates.Finally, the superexchange approach to estimate the donor acceptor electronic coupling in the stacks GAGand GTG is considered.
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