Local density calculations of CO bound to various cobalt clusters are described with the aim of determining the chemistry of the cluster as accurately as possible, and subsequently translating this into the chemistry of the metal surface. Highlighted is the importance of using an invariant cluster to compare binding site characteristics. Variations in the binding energy of CO to the clusters can be most readily ascribed to variations in cluster size and shape. Differences in energy are due primarily to the "cluster chemistry, " i.e., different responses of the cluster electrons to the adsorbate, rather than differences in the cluster-adsorbate interaction as such. An important factor is the coordination sphere of the cluster atoms; at least one should have a bulklike coordination, others must have a lower coordination to simulate the surface. Thus it is important to construct a single cluster to study adsorbate-adsorbent interactions, in which the cluster atom coordination spheres are taken into account.