A major stabilizing factor of the excimer energies comes from super-exchange. Formally, super-exchange originates from the mutual charge transfer, |Ψ 𝑋𝑋 + • Ψ 𝑌𝑌 -> and |Ψ 𝑋𝑋 -• Ψ 𝑌𝑌 + > , between two monomers. However, there is no net charge transfer in the excimer complex of pentacene nor in the stacked configuration of pentacene dimer because of symmetry. Superexchange refers to the resonance stabilization of these two CT states by mixing with the exciton states. This necessarily lowers the excimer energy relative to the corresponding exciton state because of the variational principle, which is clearly confirmed by the numerical results in Table . In the pentacene dimers, super-exchange stabilizes the 𝐿𝐿 𝑎𝑎 state significantly by -0.7 eV in the excimer and stacked dimer configurations, but to a much smaller extent (-0.09 eV) in the fishbone arrangement (Table ). Thus, phase matching of the wave functions favoring strong overlap is important to resonance mixing of inter-fragment charge transfer. In all cases, super-exchange produced no effect on the stability of the 𝐿𝐿 𝑏𝑏 state, but it does stabilize the ground state to a much smaller extent than the 𝐿𝐿 𝑎𝑎 excited states.