title: Assessment of the accuracy of coupled cluster perturbation theory for open-shell systems. I. Triples expansions.
abstract: The accuracy at which total energies of open-shell atoms and organic radicals may be calculated is assessed for selected coupled cluster perturbative triples expansions, all of which augment the coupled cluster singles and doubles (CCSD) energy by a non-iterative correction for the effect of triple excitations. Namely, the second- through sixth-order models of the recently proposed CCSD(T-n) triples series [J. J. Eriksen et al., J. Chem. Phys. 140, 064108 (2014)] are compared to the acclaimed CCSD(T) model for both unrestricted as well as restricted open-shell Hartree-Fock (UHF/ROHF) reference determinants. By comparing UHF- and ROHF-based statistical results for a test set of 18 modest-sized open-shell species with comparable RHF-based results, no behavioral differences are observed for the higher-order models of the CCSD(T-n) series in their correlated descriptions of closed- and open-shell species. In particular, we find that the convergence rate throughout the series towards the coupled cluster singles, doubles, and triples (CCSDT) solution is identical for the two cases. For the CCSD(T) model, on the other hand, not only its numerical consistency, but also its established, yet fortuitous cancellation of errors breaks down in the transition from closed- to open-shell systems. The higher-order CCSD(T-n) models (orders n > 3) thus offer a consistent and significant improvement in accuracy relative to CCSDT over the CCSD(T) model, equally for RHF, UHF, and ROHF reference determinants, albeit at an increased computational cost.
authors.affiliation: qLEAP Center for Theoretical Chemistry, Department of Chemistry, Aarhus University, DK-8000 Aarhus C, Denmark.
authors.forename: Janus J
authors.initials: JJ
authors.lastname: Eriksen
authors.affiliation: The Institute for Computational Engineering and Sciences, The University of Texas at Austin, Austin, Texas 78712, USA.
authors.forename: Devin A
authors.initials: DA
authors.lastname: Matthews
authors.affiliation: qLEAP Center for Theoretical Chemistry, Department of Chemistry, Aarhus University, DK-8000 Aarhus C, Denmark.
authors.forename: Poul
authors.initials: P
authors.lastname: Jørgensen
authors.affiliation: Institut für Physikalische Chemie, Johannes Gutenberg-Universität Mainz, D-55128 Mainz, Germany.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Gauss