Accuracies and system scaling properties have been assessed for a number of electron correlation methods including truncated Configuration Interaction (CI: CISD (with and without Davidson (+Q) correction), CISDTQ), Møller-Plesset second-order perturbation theory (MP2), Coupled Electron Pair Approximation (CEPA: AQCC, ACPF, CEPA(0)), and coupled-cluster (CC: CCSD, CCSD(T), CR-CC(2,3)). This has been achieved by (i) comparison against ~Full CI results for four small molecules, and (ii) charting recovered correlation energies as these molecules are systematically enlarged via chain extension. It is found that the triples-corrected CC methods and CEPA(0) consistently perform very similarly, and are the most superior practical methods. Additionally, all theories demonstrated proper size extensivity except (unsurprisingly) the truncated CI and CISD+Q methods. Via generation of valence virtual orbitals (VVOs), we were able to probe the recoveries of internal correlation energies where it was found that CEPA(0) consistently recovers the most, and generally behaves like Full CI. These findings led to development of the Hybrid Correlation Energy (HyCE) approach, which is based on the notion that internal and external correlation energies appear to be separable, and thus distinctly calculable. Our HyCE approach, using CEPA(0) and second-order perturbation theory for recoveries of internal and external correlation energies, respectively, closely tracks triples-corrected CC for (i) the aforementioned chain molecules and (ii) a new correlation benchmark test bed of 35 molecules. While it is generally known that electron correlation has strong (non-dynamic) and weak (dynamic) components, we show (perhaps for the first time) how this aspect can be exploited to reduce the cost of correlation energy recovery.

A comprehensive assessment of accuracies and system scaling properties of electron correlation procedures, leading to a new hybrid correlation energy (HyCE) approach

255th National Meeting (2018)

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