Ab initio molecular orbital and density functional calculations have been carried out to investigate the adsorption of CO on the Si(100)-2 × 1 surface using the Si9H12 and Si13H20 cluster models of the surface. It was found that B3LYP/6-31G(d) is a reasonable level of theory for calculation of the geometries of the clusters and adsorbates, as well as energetics of the adsorbates of the CO/Si(100)-2 × 1 surface. The addition of a doubly contracted polarizarion d-function for the non-hydrogen atoms changes the calculated CO desorption energy by 1 kcal/mol. Increasing the size of the cluster from Si9H12 to Si13H20, in general, increases the CO desorption energy by 1-2 kcal/mol, while it does not change the Sid-Sid, Sid-Sisub, and Sisub-Sisub bond distances, which suggests that the Si9H12 cluster is a good model for the single-dimer cluster. Interaction of the CO molecule with the surface dramatically changes the Sid-Sid and Sid-Sisub bond distances corresponding to the silicon dimer on the surface and that between the first- and second-layer atoms, respectively. These results suggest that the geometry relaxation of the cluster upon interaction with gas molecules should be taken into account. Different adsorption geometries of CO on the silicon surface dimer have been studied. The adsorbed CO is most stable when bonded perpendicularly to the surface dimer with the C atom attached to one of the Si atoms. The calculated CO desorption energy at the B3LYP/6-311G(2d) level, 10.5 kcal/mol, is in good agreement with the experimental value, 11.4 kcal/mol. Vibrational frequencies of the different CO adsorption isomers have been analyzed. For the OC-normal adsorption process, an extensive search for its transition state failed to locate it; this suggests that the adsorption reaction is a nonactivated process with zero barrier.