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 C-2, C-2(-), CN, CN-, O-2, and O-2(-) 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 O-2 (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 C-2(-) 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 O-2/O-2(-) and CN/CN-. For C-2/C-2(-), the observed discrepancies can be explained in terms of the well-known multiconfigurational nature of the ground state of the C-2 molecule. (C) 2001 American Institute of Physics.