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Understanding the interaction of organic molecules with TiO2 surfaces isimportant for a wide range of technological applications. While densityfunctional theory (DFT) calculations can provide valuable insight about theseinteractions, traditional DFT approaches with local exchange-correlationfunctionals suffer from a poor description of non-bonding van der Waals (vdW)interactions. We examine here the contribution of vdW forces to the interactionof small organic molecules (methane, methanol, formic acid and glycine) withthe TiO2 (110) surface, based on DFT calculations with the optB88-vdWfunctional. The adsorption geometries and energies at different configurationswere also obtained in the standard generalized gradient approximation (GGA-PBE)for comparison. We find that the optB88-vdW consistently gives shorter surfaceadsorbate-to-surface distances and slightly stronger interactions than PBE forthe weak (physisorbed) modes of adsorption. In the case of strongly adsorbed(chemisorbed) molecules both functionals give similar results for theadsorption geometries, and also similar values of the relative energies betweendifferent chemisorption modes for each molecule. In particular both functionalspredict that dissociative adsorption is more favourable than molecularadsorption for methanol, formic acid and glycine, in general agreement withexperiment. The dissociation energies obtained from both functionals are alsovery similar, indicating that vdW interactions do not affect the thermodynamicsof surface deprotonation. However, the optB88-vdW always predicts strongeradsorption than PBE. The comparison of the methanol adsorption energies withvalues obtained from a Redhead analysis of temperature programmed desorptiondata suggests that optB88-vdW significantly overestimates the adsorptionstrength, although we warn about the uncertainties involved in suchcomparisons.

Adsorption of organic molecules at the TiO2(110) surface: the effect of van der Waals interactions