On the performance of the Kohn–Sham orbital approach in the calculation of electron transfer parameters. The three state model

We have tested the performance of the Kohn–Sham orbital approach to obtain the electronic coupling and the energetics for hole transfer (HT) in the guanine–indole pair, using a three-state model. The parameters are derived from the simple DFT calculations with 10 different functionals, and compared with benchmark MS-CASPT2 calculations. The guanine–indole pair is a simple model for HT in DNA–protein complexes, which has been postulated as a protection mechanism for DNA against oxidative damage. In this pair, the first excited state of the indole radical cation has low energy (less than 0.3 eV relative to the ground state of the cation), which requires the application of very accurate quantum chemical methods and the invocation of a 3-state model. The Kohn–Sham orbital approach has been tested on six π stacked and three T-shaped conformers. It has been shown to provide quite accurate results for all ten tested functionals, compared to the reference MS-CASPT2 values. The best performance has been found for the long-range corrected CAM-B3LYP functional. Our results suggest that the Kohn–Sham orbital method can be used to estimate the excited state properties of radical cation systems studied using transient spectroscopy. Because of its accuracy and its low computational cost, the approach allows one to calculate relatively large models and to account for the effects of conformational dynamics on HT between DNA and a protein environment.