Ja. Sordo, Performance of CCSDT for first row AB/AB(-) diatomics: Dissociation energies and electron affinities, J CHEM PHYS, 114(5), 2001, pp. 1974-1980
CCSDT/aug-cc-pVXZ (X=D,T,Q) calculations were performed on the C-2, C-2(-),
CN, CN-, O-2, and O-2(-) first row diatomic molecules. The inclusion of di
ffuse functions improves the dissociation energies of the anionic systems b
y 2.0-3.4 kcal/mol, which is relevant bearing in mind the goal of achieving
chemical accuracy. The contribution of the diffuse functions in the case o
f neutral O-2 (0.6 kcal/mol) is by no means negligible in this context. A s
erious discrepancy between the theoretical prediction and the experimental
values available for the dissociation energy of C-2(-) was found. Since the
theoretical deficiences commonly ascribed to the CCSDT method (single-refe
rence and spin contamination when using UHF zeroth-order wave functions) ca
nnot be invoked in this case, further experimental work is required to thro
w some light on the origin of such a discrepancy. The performance of CCSDT
for adiabatic electron affinities is excellent in the case of O-2/O-2(-) an
d CN/CN-. For C-2/C-2(-), the observed discrepancies can be explained in te
rms of the well-known multiconfigurational nature of the ground state of th
e C-2 molecule. (C) 2001 American Institute of Physics.