  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. 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.
