  We extend our assessment of the potential of perturbative coupled cluster
(CC) expansions for a test set of open-shell atoms and organic radicals to the
description of quadruple excitations. Namely, the second- through sixth-order
models of the recently proposed CCSDT(Q-n) quadruples series [J. Chem. Phys.
140, 064108 (2014)] are compared to the prominent CCSDT(Q) and lambda-CCSDT(Q)
models. From a comparison of the models in terms of their recovery of total CC
singles, doubles, triples, and quadruples (CCSDTQ) energies, we find that the
performance of the CCSDT(Q-n) models is independent of the reference used
(unrestricted or restricted (open-shell) Hartree-Fock), in contrast to the
CCSDT(Q) and lambda-CCSDT(Q) models, for which the accuracy is strongly
dependent on the spin of the molecular ground state. By further comparing the
ability of the models to recover relative CCSDTQ atomization energies, the
discrepancy between them is found to be even more pronounced, stressing how a
balanced description of both closed- and open-shell species - as found in the
CCSDT(Q-n) models - is indeed of paramount importance if any perturbative CC
model is to be of chemical relevance for high-accuracy applications. In
particular, the third-order CCSDT(Q-3) model is found to offer an encouraging
alternative to the existing choices of quadruples models used in modern
computational thermochemistry, since the model is still only of moderate cost,
albeit markedly more costly than, e.g., the CCSDT(Q) and lambda-CCSDT(Q)
models.
