English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

The accuracy at which total energies of open-shell atoms and organic radicalsmay be calculated is assessed for selected coupled cluster perturbative triplesexpansions, 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 proposedCCSD(T-n) triples series [J. Chem. Phys. 140, 064108 (2014)] are compared tothe acclaimed CCSD(T) model for both unrestricted as well as restrictedopen-shell Hartree-Fock (UHF/ROHF) reference determinants. By comparing UHF-and ROHF-based statistical results for a test set of 18 modest-sized open-shellspecies with comparable RHF-based results, no behavioral differences areobserved for the higher-order models of the CCSD(T-n) series in theircorrelated descriptions of closed- and open-shell species. In particular, wefind that the convergence rate throughout the series towards the coupledcluster singles, doubles, and triples (CCSDT) solution is identical for the twocases. For the CCSD(T) model, on the other hand, not only its numericalconsistency, but also its established, yet fortuitous cancellation of errorsbreaks down in the transition from closed- to open-shell systems. Thehigher-order CCSD(T-n) models (orders n>3) thus offer a consistent andsignificant improvement in accuracy relative to CCSDT over the CCSD(T) model,equally for RHF, UHF, and ROHF reference determinants, albeit at an increasedcomputational cost.

Assessment of the accuracy of coupled cluster perturbation theory for open-shell systems. I. Triples expansions