Ab iliitio self-consistent field and complete active space self-consistent field cluster-model wave func­tions have been obtained for a CO-Pt4 cluster model simulating the atop interaction of CO on Pt(lll). The origin of the vibrational shift between free and chemisorbed CO has been investigated by means of the constrained space orbital variation method. This analysis shows that the vibrational shift is the re­sult of severa} effects. First, there is a large positive shift due to Pauli repulsion, and second various neg­ative contributions; these are substrate polarization, u donation, and 1r back donation, respectively. This theoretical analysis shows that the mechanism suggested by Blyholder is, in fact, the one responsible for the observed vibrational shift.