Fast and Accurate Charge Transfer Excitations via Nested Aufbau Suppressed Coupled Cluster

Modeling charge transfer well can require treating post-excitation orbital relaxations and handling medium to large molecules in realistic environments. By combining a state-specific correlation treatment with such orbital relaxations, Aufbau suppressed coupled cluster has proven accurate for charge transfer, but, like many coupled cluster methods, it struggles with large system sizes. We derive a low-cost Aufbau suppressed second order perturbation theory and show that, by nesting a small coupled cluster treatment inside of it, computational cost and scaling are reduced while accuracy is maintained. Formal asymptotic costs are dropped from iterative $N^6$ to non-iterative $N^5$ plus iterative $N^3$, and we test an initial implementation that can handle about 100 atoms and 800 orbitals on a single computational node. Charge transfer excitation energy errors are typically below 0.1 eV on average, with an average 0.25 eV improvement over $N^6$-cost equation of motion coupled cluster with singles and doubles.