Calculations were presented for Ti adatoms and interstitials at the (110) surface of rutile TiO2, where these species were known to play a crucial role in surface chemistry. A review was presented of structural calculations performed using the DFT+U methodology, which were benchmarked using controlled self-doping experiments on thin rutile films. The ab initio results were further used to assess the ability of empirical charge equalisation (QEq) potentials to correctly predict the energetics of these structures. A simple modification to the potential, whereby the oxygen charge was fixed while allowing charge redistribution between Ti ions, was shown to greatly improve its performance in terms of the energy landscape of the Ti adatoms and interstitials. The QEq calculations were here extended to consideration of the diffusion pathways and barriers in the surface using nudged elastic band calculations. It was found that key barriers involved in the diffusion of Ti interstitials to adatom sites were much lower with the modified potential, implying that the diffusion was active on experimental time scales at temperatures where the re-growth of reduced rutile crystals exposed to oxygen was observed.

Adatoms and Interstitials in the Rutile TiO2 (110) Surface: Structure and Dynamics. Mulheran, P.A., Browne, C.S., Moghaddam, Y.: Molecular Simulation, 2009, 35[7], 532-7