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

Modeling charge transfer well can require treating postexcitation 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 to be 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 N⁶ to noniterative N⁵ plus iterative N³, 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, with an average 0.25 eV improvement over the N⁶-cost equation of motion coupled cluster with singles and doubles.