The convergence of the binding energy, geometry, and vibrational frequencies for CO/Cu(100) is studied using a cluster model containing up to 38 Cu atoms. A large basis set is used for the Cu atom at the adsorption site and for the CO molecule. Electron correlation, which is included at the modified coupled-pair functional (MCPF) level, is found to increase the binding energy by about 1 eV. The binding energy and geometry are in very good agreement with experiment for the largest Cu clusters considered. The MCPF binding energies for the smaller clusters can be either larger or smaller than experiment. Unlike the binding energies, the shift in the CO vibrational frequency, relative to foe CO, is quite independent of the cluster size or even the inclusion of electron correlation.