Periodic density functional theory (DFT) based on plane waves (PW) and Hartree-Fock (HF) based on the linear combination of atomic orbitals (LCAO) calculations using slabs separated by vacuum gaps were carried out to model the H ₂O-TiO ₂ (rutile) (110) interface. Positions of all atoms were allowed to relax except atoms in the central layer of the slab. Both associative and dissociative adsorption mechanisms were considered for half-monolayer and monolayer coverages. Five different orientations of H ₂O molecules on the TiO ₂ surface were studied to determine the most energetically favorable water positions for the associative mechanism. Two slab thicknesses (three Ti layers and five Ti layers) were chosen to test the effect of slab depth on calculated surface structures and adsorption energies. Results indicate that associative adsorption is favorable by -8 to -20 kJ/mol/H ₂O depending on the slab thickness for full-monolayer coverage. Embedded cluster HF calculations were also performed for comparison. Adsorption energies of H ₂O in the embedded cluster case are much more favorable for the associative mechanism. The role of H-bond formation on the adsorption energies and structures is discussed.