A THEORETICAL-STUDY OF CO CU(100)

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