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 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.