We have developed a variety of reduced-cost, highly parallelizable electronic structure models for describing collective excitations in molecular liquids, crystals, and aggregates. Some of these represent low-cost approximations to the Casida equations in time-dependent density functional theory, while another represents an ab initio implementation of the Frenkel-Davydov ansatz, in which a collective excitation is expanded in a direct-product basis of monomer excitations. The monomer wave functions and the couplings between them can be computed in a trivially-parallelizable fashion, and calculations equivalent to more than 55,000 basis functions have been performed in a few days on only a few hundred processors. Analytic derivative couplings have been derived and implemented for this ab initio Frenkel-Davydov model, and can be used to identify the vibrational modes that promote intermolecular energy transfer. The derivative couplings are equivalent to electron/phonon couplings that appear as parameters in vibronic model Hamiltonians, and we have parameterized such a model to describe the singlet fission process in crystalline tetracene. Quantum dynamics simulations using this Hamiltonian demonstrate that vibronic coherence can explain why singlet fission is fast in efficient in tetracene, despite ostensibly unfavorable electronic energetics.

First-principles exciton models, with application to singlet fission

254th National Meeting (2017)

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