We present calculated surface and interstitial transition barriers for Ti, O, O 2 , TiO, and TiO 2 atoms and clusters at the rutile ͑110͒ surface. Defect structures involving these small clusters, including adcluster and interstitial binding sites, were calculated by energy minimization using density-functional theory ͑DFT͒. Transition energies between these defect sites were calculated using the NEB method. Additionally, a modified SMB-Q charge equilibration empirical potential and a

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... ge empirical potential were used for a comparison of the transition energy barriers. Barriers of 1.2-3.5 eV were found for all studied small cluster transitions upon the surface except for transitions involving O 2 . By contrast, the O 2 diffusion barriers along the ͓001͔ direction upon the surface are only 0.13 eV. The QEq charge equilibration model gave mixed agreement with the DFT calculations, with the barriers ranging between 0.8 and 5.8 eV.