Performance of CCSDT for first row $AB/AB−$ diatomics: Dissociation energies and electron affinities

CCSDT/aug-cc-pVXZ $(X=D,T,Q)$ calculations were performed on the $C2,$ $C2−,$ CN, $CN−,$ $O2,$ and $O2−$ first row diatomic molecules. The inclusion of diffuse functions improves the dissociation energies of the anionic systems by 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 of neutral $O2$ (0.6 kcal/mol) is by no means negligible in this context. A serious discrepancy between the theoretical prediction and the experimental values available for the dissociation energy of $C2−$ was found. Since the theoretical deficiences commonly ascribed to the CCSDT method (single-reference and spin contamination when using UHF zeroth-order wave functions) cannot be invoked in this case, further experimental work is required to throw some light on the origin of such a discrepancy. The performance of CCSDT for adiabatic electron affinities is excellent in the case of $O2/O2−$ and $CN/CN−.$ For $C2/C2−,$ the observed discrepancies can be explained in terms of the well-known multiconfigurational nature of the ground state of the $C2$ molecule.