Fragment-Based Configuration Interaction: Toward a Unifying Description of Biexcitonic Processes in Molecular Aggregates

delivers benchmark-quality couplings. The resulting diabatic Hamiltonians can be coupled to quantum dynamics simulations. Applications to ethylene aggregates and the anthracene crystal reveal CTX configurations as electronic gateways bridging excitonic manifolds, with CT-mediated relaxation pathways competing with conventional annihilation. In H-type aggregates, the LECT admixture stabilizes a "biexcimer" analogous to one-particle excimers. By providing first-principles access to biexciton formation, separation, and transport, we hope to stimulate further exchange between electronic-structure and quantum dynamics communities toward a predictive understanding of multiexcitonic photophysics.