Exothermic water dissociation on the rutile<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">Ti</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mn>110</mml:mn><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math>surface

There has been a long-running debate among theorists and experimentalists on the precise nature of water adsorption at the $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ surface. Some experimentalists argue that dissociative adsorption occurs only at defect sites (O vacancies) and therefore at low coverages. Although there is no doubt that vacancies are strongly reactive, until now there has been no firm understanding of adsorption on a perfect surface with which to contrast behavior. Here we report extensive and very detailed calculations that demonstrate that dissociation of a molecule is exothermic. Experimental findings are rationalized by the existence of a metastable molecular state separated from the dissociated state by a substantial barrier. We show that the barrier varies in height with coverage and with the presence of neighboring adsorbates, and we detail mechanisms for both phenomena. Finally, we reassess photoelectron spectroscopy results, showing their consistency with our predictions.