On the behavior of different contributions to the excitonic coupling at short intermolecular separations

The treatment of large molecules by high-level quantum chemical methods requires either localization schemes or multilevel methods. However, the description of multichromophore systems poses a challenge for these schemes due to the spatial separation of the excitation events. An often applied technique is based on the fragmentation of the system with the inclusion of excitonic coupling between the fragments holding the chromophores. In this paper, we discuss some aspects of this latter methodology. Three contributions to the excitonic coupling will be discussed. The Coulomb coupling is the dominating component at large distances and is often used as the sole contribution. The exchange term is sometimes also evaluated, but it seems to be important only at short distances. Finally, there is also a term arising from the overlap of the fragment wave functions, which needs to be included when diagonalizing the Hamiltonian of coupled excitons. In this study, we investigate the distance dependence of these terms for π-stacked molecular pairs and evaluate their relative importance. We also investigate how the numerical value of these contributions depends on the level of quantum chemical calculation (CIS, CC2, or EOM-CCSD) and also show the role of diffuse basis functions.