We have studied the strong variation of cluster-model binding energies of the CO/Cu(100) adsorbate system for clusters with between 1 and 34 atoms to represent the Cu surface. Based on the constrained space orbital variation approach the metal-CO interaction is decomposed into three terms: (1) charge superposition of the free CO and metal subunits, (2) charge polarization within the subunits, and (3) charge transfer between the subunits. The results show that one can identify bonding contributions which vary slowly with cluster size and shape. Further, relations between the cluster representation of the Cu(100) conduction band and the metalco bond strength can be established. These relations make it possible to determine the importance of cluster artifacts for the metalco bonding from electronic properties of the bare metal cluster. Our results provide further support for the validity of using clusters with small numbers of substrate atoms to describe chemisorption on metal surfaces. © 1987 The American Physical Society.