Intramolecular photo-induced charge transfer in visual retinal chromophore mimics: electron density-based indices at the TD-DFT and post-HF levels

The first bright transition of a series of ten retinal-based dyes was simulated using TD-DFT, with several hybrid functionals, and with several post-Hartree–Fock methods. The CASPT2 results were used as reference to compare the results. The comparisons were not only based on the transitions energies, that is generally the case for such works, but were also based on the variation of the electron density from the ground to the excited along with several density-based indices recently developed. Among all the results obtained, it clearly appears that the PT2 correction to the CASSCF wavefunction is increasing with the increase in the charge transfer character of the transition. As already highlighted in the literature, TD-DFT poorly reproduces transition energies of this family of molecules whatever the functional. This work also confirms this observation for the variation of the electron density that is different between TD-DFT and CASPT2. Nevertheless, some functionals give better results than others. It appears that functionals having a fraction of 40–50 % of exact exchange give a better description of the electron density variation than the other functionals. Moreover, the best functional to compute the transition energies is not the best to simulate excited electron density that confirms an observation done in the literature on a different family of molecules.