A systematic study of the effect of the slab thickness on the Lewis acidity of the rutile (1 1 0) surface has been performed by simulating the adsorption of carbon monoxide. The attention has been focused on the molecule-surface distance, the binding and interaction energies and the adsorbed CO stretching frequency. The study shows that converging data, being in good agreement with the experimental ones, are achieved by adopting at least a 12-atomic layers slab and that the use of the possible thinnest slab does not lead to reliable results. (C) 2009 Elsevier B.V. All rights reserved.