CLUSTER MODEL DESCRIPTION OF THE CHEMISORPTION BOND - EFFECT OF THE CLUSTER MODEL ELECTRONIC-STATE

Chemisorption of CO on Cu(100) has been modeled by a Cu5CO cluster. Ab initio (SCF) electronic wavefunctions have been obtained for the two low-lying electronic states which, in principle, have different bonding capabilities. Potential energy curves show that in one case the interaction is repulsive whereas in the other case a minimum is found. However, an analysis of the interaction in terms of constrained variations shows that the nature of the bond is essentially the same for both states; the main difference arises from the initial Pauli or steric repulsion of the superimposed substrate and adsorbate charge distributions. The differences between the two states can be ascribed to the limited cluster representation of the surface conduction band. An important consequence of this limitation is that great care must be taken to obtain estimates of the chemisorption bond energy. However, because the basic bond mechanism may be the same for different cluster states as well as for different cluster sizes, it is easier to obtain reasonable estimates of structural parameters such as bond distances and vibrational frequencies.