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.

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