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.
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(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.