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We develop a new first-principles approach to predict and understand rates ofsinglet fission with an ab initio Green's-function formalism based on many-bodyperturbation theory. Starting with singlet and triplet excitons computed from aGW plus Bethe-Salpeter equation approach, we calculate the exciton--bi-excitoncoupling to lowest order in the Coulomb interaction, assuming a final stateconsisting of two non-interacting spin-correlated triplets with finitecenter-of-mass momentum. For crystalline pentacene, symmetries dictate that theonly purely Coulombic fission decay from a bright singlet state requires afinal state consisting of two inequivalent nearly degenerate triplets ofnonzero, equal and opposite, center-of-mass momenta. For such a process, wepredict a singlet lifetime of 40 to 150 fs, in very good agreement withexperimental data, indicating that this process can dominate singlet fission incrystalline pentacene. Our approach is general and provides a framework forpredicting and understanding multiexciton interactions in solids.

Origins of singlet fission in solid pentacene from an ab initio Green's-function approach