The molecular orbital cluster model has been used to study the vibrations of CO interacting with metal surfaces. In all cases the equilibrium CO geometry is taken normal to the surface with C directly above a metal atom; SCF and correlated Cl wave functions have been obtained. For CO/ Cu(100), we have studied the C-O and Cu-C stretch modes. The C-O frequency ωe and effective charge e have been analyzed in terms of σ and π contributions; the adsorption site metal π,3π, and 4pπ, donation to CO 2πcontributes in a major way to e. The ωe and e for the C-O stretch have been compared for one metal atom-CO clusters of Cu, Ni, and Fe. The ωe decrease in the order Cu, Ni, and Fe; the e increase in the same order. This is to be expected from the order of the metal dir to CO 2π donation and bonding which is Fe > Ni > Cu. © 1985, American Vacuum Society. All rights reserved.