and the first excited state; however, a noticeable stabilization to the second excited state of the excimer, by 0.19 eV, is obtained (Figure ). Overall, a net binding energy of -0.23 eV (-5.3 kcal/mol) for the acetone excimer is obtained both from NOSI and TDDFT calculations. We now turn to the case of pentacene excimer, which exhibits different behaviors in comparison with that in the asymmetric acetone excimer. Listed in Table are the computed energy terms, including local excitation Δ𝐸𝐸 Lex , exciton excitation states Δ𝐸𝐸 Ex , super-exchange stabilization Δ𝐸𝐸 SE , and the orbital and configuration delocalization energies Δ𝐸𝐸 OCD for the excimer complex, associated with the excimer states of pentacene in three different geometrical arrangements. The first geometry corresponds to a fully optimized structure of the 𝑆𝑆 1 excimer (D2h symmetry) with the shortest inter-fragment separation at 3.15 Å, the second configuration is an on-top stacked construction with the ground-state monomer geometry at 3.3 Å, and the third is derived from the pentacene crystal in a fishbone relationship used previously (Figure ). These structures are of interest by their own rights, but the fishbone relationship is directly relevant to pentacene monolayer materials used in solar cell research. However, we shall not address the mechanism of singlet fission in the present MS-EDA study; interested readers may find conclusions in an early study using MSDFT. 67 and), respectively. This is significant in view of the non-polar nature of the hydrogen carbon system, although there is also a slight change from structural variations from the monomer to the excimer. The most striking finding from Table and also in Figure is the large exciton coupling energies and the resulting energy splitting both for the 𝐿𝐿 𝑎𝑎 and 𝐿𝐿 𝑏𝑏 states in the excimer and stacked structures, giving rise to exciton splitting energies |2Δ𝑉𝑉 Ex | of 1.13 and 0.84 eV for the 𝐿𝐿 𝑎𝑎 states, and 0.68 and 0.67 eV for the 𝐿𝐿 𝑏𝑏 states (Table ). Interestingly, the computed exciton coupling (energy splitting) between locally excited states is fortuitously nearly identical to that determined as the energy difference between the two adiabatic states (Table , Figure ). In comparison with the "solar-cell relevant" fishbone structure, the computed exciton coupling is only 0.05eV for the 𝐿𝐿 𝑎𝑎 state and 0.01 eV for the 𝐿𝐿 𝑏𝑏 state. Phase matching of the two chromophore wave functions in the dimer (excimer) complex is critical for the strength of their interaction and can have a major impact on the energy levels. This effect is clearly reflected in the tilted monomers in the fishbone complex. Thus, a small adjustment to better align the overlap of monomer wave functions may significantly enhance electronic coupling for energy and charge transfer.