Please post questions about Q-Chem Webinar 83 (presented by Manisha on Nov 19, 2025) in the thread here!
Abstract:
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.
About the Presenter: Manisha is currently a PhD Scholar in the Department of Chemistry at BITS Pilani. She obtained her B.Sc. from DAV college Jalandhar in 2017 and her M.Sc. from Mata Gujri College in 2019. She then joined the Prashant Uday Manohar group at BITS Pilani, where she is currently working on computational methods for accurately describing strongly degenerate electronic states, including implementation of equation-of-motion coupled-cluster variants with singles, doubles and full triples. She has made many contributions to the Q-Chem software over the past several years, and she joined us in the Q-Chem office this past summer as one of our 2025 Summer at Q-Chem interns!