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

Area of Science:

• Computational chemistry
• Theoretical chemistry
• Quantum chemistry

Background:

• Accurate modeling of charge transfer requires accounting for postexcitation orbital relaxations and realistic molecular environments.
• Coupled cluster methods, while accurate, face limitations in scaling to large systems.
• Previous methods like Aufbau suppressed coupled cluster show promise but struggle with system size.

Purpose of the Study:

• To develop a computationally efficient method for modeling charge transfer in medium to large molecules.
• To maintain accuracy in charge transfer calculations while significantly reducing computational cost and improving scalability.
• To provide a more accessible tool for studying charge transfer processes in realistic chemical environments.

Main Methods:

• Derivation of a low-cost Aufbau suppressed second-order perturbation theory.
• Integration of a small coupled cluster treatment within the perturbation theory framework.
• Development of an implementation capable of handling systems with approximately 100 atoms and 800 orbitals.

Main Results:

• Significant reduction in computational cost from iterative N^6 to noniterative N^5 plus iterative N^3.
• Maintained accuracy for charge transfer calculations, with typical excitation energy errors below 0.1 eV.
• Demonstrated improvement over existing N^6-cost equation of motion coupled cluster methods, with an average of 0.25 eV error reduction.

Conclusions:

• The new hybrid computational method effectively balances accuracy and computational efficiency for charge transfer modeling.
• This approach overcomes the scaling limitations of traditional coupled cluster methods for larger molecular systems.
• The developed implementation offers a practical tool for advanced theoretical studies of charge transfer phenomena.