The eight-valence-electron systems re-examined: convergence of the coupled-cluster series and performance of quasiperturbative methods for quadruple excitations

Abstract

We present the results of a coupled-cluster basis-set convergence study on 12 eight-valence-electron systems. These small systems span a wide range of correlation character (from purely dynamical correlation to pathological static correlation) as well as of chemical bonding (from ionic with some backdonation to quadruple covalent bonding). Indicators such as the correlation entropy and the CASSCF 1-C₀², which primarily sample type A static correlation, are good predictors for the energetic importance of connected quintuple excitations, T₅, and to a lesser extent of higher order triple excitations, T₃–(T). The percentage of the total atomisation energy recovered by connected triple excitations, %TAE[(T)], is however a fair predictor for the importance of connected quadruples, T₄. Of the various ansätze for quasiperturbative connected quadruples, CCSDT(Q)/B appears to be the most robust, even as it is still outperformed by the much more expensive CCSDT(Q)_Λ.

Keywords:

• coupled cluster
• nondynamical correlation
• connected quadruple excitations
• higher-order correlation effects
• convergence of the coupled cluster expansion

Acknowledgements

The author thanks Prof. Mihály Kállay (BME, Hungary) for assistance with the new version of MRCC, Dr Irena Efremenko for assistance with AIMALL, and Dr Mark Vilensky for technical assistance with the Faculty of Chemistry computing cluster. The study was inspired by a discussion with Ms Rebecca Weber and Prof. Angela K. Wilson (University of North Texas) concerning the LiO⁻ molecule.

Additional information

Funding

This research was partially supported by the Lise Meitner-Minerva Center for Computational Quantum Chemistry as well as by the Helen and Martin A. Kimmel Center for Molecular Design.