Nature of Mo-Mo and Cr-Cr multiple bonds: A challenge for the local-density approximation

The full potential curves for Mo2 (and Cr2) have been examined by using highly correlated wave functions. A competition is found between 5s-5s bonding and 4d-4d bonding, so that the lower states lead to two minima, with the inner minimum involving dσ, dπ, and dδ bonds. The results disagree dramatically with those from local-density calculations, and the present analysis suggests that this approximation (in its current formulation) is unsuitable for describing multiple bonds in transition-metal systems.