Charge transfer via spin flip configuration interaction: Benchmarks and application to singlet fission.

Charge transfer and multi-exciton states are among the most difficult to characterize using electronic structure theories. Previously, restricted active space-spin flip (RAS-SF) methods have been applied to describe multi-exciton states, but these have not yet been shown to be useful for charge-transfer states. Herein, a variant of RAS-SF is introduced to treat charge-transfer states and electronic couplings. This approach relies on partitioning of the full RAS-SF Hamiltonian into charge-transfer and non-charge-transfer blocks, allowing the different types of diabatic states to be resolved in a straightforward fashion. To demonstrate this approach in practice, model dimer systems, intramolecular charge-transfer dyads, and an intramolecular singlet fission system were examined. Being low-cost and relatively accurate, RAS-SF provides important insight into electron transfer pathways in conventional donor-acceptor systems, as well as characterizations of charge transfer mechanisms involving strongly correlated multi-exciton states. Studies of electron transfer from an intramolecular singlet fission chromophore to an anthraquinone acceptor demonstrate the unique capabilities of the proposed RAS-SF method.