Hybrid density functional calculations have been carried out for AuO2, AuCO, Au-13, Au13O2, Au13CO, Au13H2 and Au-55 clusters to discuss the catalytic behavior of Au clusters with different sizes and structures for CO oxidation. From these calculations, it was found that O-2 and CO could adsorb onto several Au model systems. Especially, icosahedral Au-13 cluster has a relatively weak interaction with O-2 while both icosahedral and cubooctahedral Au-13 clusters have interactions (similar to 20 kcal/mol) with CO. These findings suggest that the surfaces of the An clusters are the active sites for the catalytic reactions on the supported and unsupported Au catalysts. (c) 2005 Elsevier B.V. All rights reserved.