Interaction of Dark Excited States. Comparison of Computational Approaches

A systematic theoretical study of the electronic interaction of dark excited states in a model system, formaldehyde dimer is reported. Using the fragment transition density scheme, we estimate the excitonic interaction in different configurations of the dimer. The excited state properties of the system are computed with several quantum mechanical methods. We show that the orbital interaction of the monomers rather than Coulomb interaction of their transition quadrupoles gives the major contribution to the coupling at intermolecular distances shorter than 5 Å. It is found that the exitonic interaction alters drastically by conformational changes. Benchmark couplings computed with EOM CCSD, MS-CASPT2, CASSCF, TD DFT, CIS, and INDO/S and different basis sets are provided. The evaluation of the calculations shows that the TD cam-B3LYP scheme performs best, giving good estimates for all considered structures. In contrast, the TD B3LYP scheme leads to drastically overestimated values. The data obtained using the Tamm-Dancoff approximation are similar to the TD DFT results. CASSCF and CIS calculations underestimate the coupling, indicating that dynamic electron correlation may have a large effect on the short-range coupling. The INDO/S method fails to describe the excited state interaction both at short and long distances.