Energy decomposition analysis (EDA) is a useful tool for understanding intermolecular interactions. Although methods for molecular complexes in the ground state have been thoroughly developed, few approaches are currently available for analyzing energy terms of excimers and exciplexes -molecular aggregates formed in the electronic excited states. One exception is a study by Ge et al., who used the same energy terms in ground-state EDA, including frozen (frz), polarization (pol) and charge transfer (CT) terms to describe intermolecular interactions in the excited states Δ𝐸𝐸 int * = Δ𝐸𝐸 frz * + Δ𝐸𝐸 pol * + Δ𝐸𝐸 CT * . The method was introduced on the basis of configuration interaction with singly excitations (CIS), in principle, extendable to timedependent density functional theory (TDDFT) for excited-state calculations. In particular, the frozen excitation energy was obtained with the use of fixed orbitals of each isolated fragment along with its excitation amplitudes, and frozen interactions were modeled through the merged Fock matrix. Similarly, polarization contribution was determined with the relaxed, fragment blocklocalized molecular orbitals (BLMO) also called absolutely localized MOs. The CT term was simply the difference between the excitation energy of the full complex and the other two terms. That work was extended to excimers with the addition of an exciton splitting term between the frozen excited states. That work provided an analysis of excited state interactions from a ground-state analogy. On the other hand, it is of interest to define variationally optimizable diabatic states, such as the local excitations of individual chromophores in the presence of other molecules in the ground stat. Electronic coupling interaction among local states can be used to determine the rates of excited-state energy transfer such as light-harvesting in photosynthesis and perception, photoexcitation-induced oxidation-reduction reactions and processes in photovoltaic devices and fuel cells. In this Letter, we introduce an alternative, multistate energydecomposition analysis (MS-EDA) for the interaction energy of an excimer complex -excited-state complex formed between the same chemical species. We focus on energy terms unique to excited states, including local excitation, exciton excitation Δ𝐸𝐸 𝐸𝐸𝐸𝐸 , resonance stabilization due to super-exchange ΔE 𝑆𝑆𝐸𝐸 , and orbital and configuration-state delocalization Δ𝐸𝐸 𝑂𝑂𝑂𝑂𝑂𝑂 . The method is illustrated on the acetone and pentacene excimers, relevant to chemiluminescence and singlet fission in materials for solar cells. Considering the following photochemical process of excimer formation,