The potential energy surface ( PES) describing the diffusion and vibration of CO molecules adsorbed on a Cu(100) surface has been calculated using density functional theory within two different generalized gradient approximations ( GGAs), employing a slab representation of the surface. One goal of the study was to investigate the origin of the discrepancy between a recently published PES [ J. Chem. Phys. 114, 1053 ( 2001)] and inferences that had been made from various experiments. A further aim was to create a PES that could provide a better basis for modeling and understanding of the diffusive and vibrational motion in the CO/ Cu(100) system. We found that the calculated PES has a global minimum on- top of the substrate atoms in line with the experimentally determined adsorption site. Diffusion is preferred across the bridge site, a saddle point, with a diffusion barrier of 95+/-30 meV and 125+/-30 meV for the two GGAs. Vibrational frequencies deduced from the PES agree with experimental results to within 10 meV. (C) 2003 American Institute of Physics.