Gustavo Ribeiro Aroeira Graduate Student, Department of Chemistry University of Georgia Learn more about the speaker Friday, February 26, 2021 - 11:30am ONLINE ONLY Physical Seminar An externally corrected coupled cluster (CC) 1 method, where an adaptive configuration interaction (ACI) 2,3 wave function provides the external cluster amplitudes, named ACI-CC, was presented. By exploiting the connection between configuration interaction and coupled cluster through cluster analysis, the higher-order T3 and T4 terms obtained from ACI are used to augment the T1 and T2 amplitude equations from traditional coupled cluster. These higher-order contributions are kept frozen during the coupled cluster iterations and do not contribute to an increased cost with respect to CCSD. We have benchmarked this method on three closed-shell systems: beryllium dimer, formaldehyde oxide, and cyclobutadiene, with good results compared to other corrected coupled cluster methods. 4 In all cases, the inclusion of these external corrections improved upon the “gold standard” CCSD(T) results, indicating that ACI-CCSD(T) can be used to assess strong correlation effects in a system and as an inexpensive starting point for more complex external corrections. Future work will assess the extension of the ACI wave function and generalizations for open shell systems. References (1) Paldus, J. Externally and internally corrected coupled cluster approaches: an overview. J. Math. Chem. 2016, 55, 477–502. (2) Schriber, J. B.; Evangelista, F. A. Communication: An adaptive configuration interaction approach for strongly correlated electrons with tunable accuracy. J. Chem. Phys. 2016, 144, 161106. (3) Schriber, J. B.; Evangelista, F. A. Adaptive Configuration Interaction for Computing Challenging Electronic Excited States with Tunable Accuracy. J. Chem. Theory Comput. 2017, 13, 5354–5366. (4) Li, X.; Paldus, J. Accounting for the exact degeneracy and quasidegeneracy in the automerization of cyclobutadiene via multireference coupled-cluster methods. J. Chem. Phys. 2009, 131, 114103.