Coupled Cluster Externally Corrected by Adaptive Configuration Interaction

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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