Excited-state energy decomposition analysis (EDA) provides a useful framework to dissect the physical interactions that stabilize molecular complexes in electronically excited states. While ground-state EDA has been widely applied to understand noncovalent interactions and chemical bonding, excited states introduce qualitatively new contributions, including photoexcitation, exciton resonance, and charge-transfer excitations. Recent developments in multistate density functional theory (MSDFT) extends the interpretability of EDA into the excited-state domain, offering mechanistic insight into photophysical and photochemical phenomena. This report summarizes the theoretical foundation of multistate EDA (MS-EDA), defines its key energetic terms, and illustrates its application to several groups of excited-state complexes. It is hoped that MS-EDA can provide interpretable understanding of excited state energies in terms of exciton resonance, superexchange stabilization and orbital and configuraiton delocalization.
