Webinar 83: Recent Coupled Cluster Developments in Q-Chem

This webinar was presented by Manisha on November 19, 2025. The inclusion of triple excitations in the coupled-cluster expansion is essential for capturing higher-order electron correlation effects beyond the reach of singles and doubles. Triples provide a balanced and physically consistent description of systems with strong correlation, significantly improving total energies, potential energy surfaces, and the qualitative behavior of molecular dissociation. In the first part of this presentation, I will outline our recent implementations within Q-Chem, including the spin-flip (SF), ionization potential (IP), electron attachment (EA), double ionization potential (DIP), and double electron attachment (DEA) EOM-CCSDT variants. These developments extend the applicability of coupled-cluster theory to multiconfigurational electronic states, offering accurate and systematically improvable descriptions of challenging electronic structures. In the second part, I will discuss the development of the CCSDt method, which was carried out as part of my Summer@Q-Chem internship. The CCSDt scheme introduces a hierarchy of approximations to the standard CCSDT model, where the dominant T3 contributions are evaluated using the concept of active-space orbitals. This design enables the method to achieve computational scaling of approximately no2nu4 while retaining size-extensivity and maintaining reliability for quasi-degenerate systems.