A benchmark of excitonic couplings derived from atomic transition charges.

In this report we benchmark Coulombic excitonic couplings between various pairs of chromophores calculated using transition charges localized on the atoms of each monomer chromophore, as derived from a Mulliken population analysis of the monomeric transition densities. The systems studied are dimers of 1-methylthymine, 1-methylcytosine, 2-amino-9-methylpurine, all-trans-1,3,5-hexatriene, all-trans-1,3,5,7-octatetraene, trans-stilbene, naphthalene, perylenediimide, and dithia-anthracenophane. Transition densities are taken from different single-reference electronic structure excited state methods: time-dependent density functional theory (TDDFT), configuration-interaction singles (CIS), and semiempirical methods based on intermediate neglect of differential overlap. Comparisons of these results with full ab initio calculations of the electronic couplings using a supersystem are made, as are comparisons with experimental data. Results show that the transition charges do a good job of reproducing the supersystem couplings for dimers with moderate to long-range interchromophore separation. It is also found that CIS supermolecular couplings tend to overestimate the couplings, and often the transition charges approach may be better, due to fortuitous cancellation of errors.