Modeling Triplet Excited-State Energy Transfer (TEET) via Multistate Density Functional Theory with Nonorthogonal State Interaction (MSDFT–NOSI)

We present a computational protocol for triplet excited-state energy transfer (TEET) within multistate density functional theory with nonorthogonal state interaction (MSDFT-NOSI). Block-localized excitation, achieved through occupation-constrained orbital optimization, generates fragment-localized singlet and triplet configurations in a minimal active space that captures donor and acceptor states. Well-defined diabatic states, with excitations localized on individual fragments, are obtained via the generalized diabatic-at-construction (GDAC) transformation. This yields diabatic energies and electronic couplings for both Dexter-type and Förster-type transfer in a unified framework. Application to the propenal excimer demonstrates that MSDFT-NOSI reproduces TDDFT-quality excitation energies while providing a chemically transparent picture of the TEET process. MSDFT-NOSI/GDAC offers a practical and insightful method for investigating triplet energy transfer in molecular complexes and photocatalytic systems.