The CO-CO adsorbate interaction on Si(100)-2×1 has been investigated with ab initio molecular orbital and hybrid density functional theory calculations using cluster models of the surface. Different adsorption combinations for one and two CO molecules on single- and double-dimer cluster models, Si9H12 and Si15H16, respectively, are described. Our calculations indicate that the second CO molecule is physisorbed on the same surface Si dimer where the first CO molecule is chemisorbed. The chemisorption of the first CO molecule induces a change in the charge of the surface Si dimer atoms which inhibits further adsorbate-surface interaction. The dissociation energy of the physisorbed second CO molecule is less than 1 kcal/mol. Adsorption of the second CO molecule on the second Si dimer is energetically preferred over coadsorption of CO on the same Si dimer. The 2OC-normal.d14 structure is the most stable configuration, with the two CO molecules adsorbed diagonally across the two Si dimers. The dissociation energy of the chemisorbed second CO molecule in the 2OC-normal.d14 structure is 13.8 kcal/mol, about 4 kcal/mol less stable than the first adsorbed CO. Adsorption of two CO molecules in a bridge configuration indicates a weak surface-adsorbate and/or adsorbate-adsorbate interaction. The dissociation energy of the single chemisorbed CO in the bridged state is 5.0 kcal/mol while that of the second CO is 1.7 kcal/mol. A mixed configuration, i.e., OC-normal with OC-bridge, was found to be unstable based on the Si15H16 surface model.