Computational details: Calculations were performed using the PBE-GGA exchange and correlation functional 1 . Electron-core interactions were described by ultrasoft 2 pseudopotentials, treating O (2s, 2p) and Ti(3s, 3p, 3d and 4s) shells explicitly. A kinetic energy cutoff of 20 Ry was used for the plane-wave basis and a cutoff of 200 Ry for the augmented density. In the GGA calculations the bottom layer was fixed at its relaxed bulk coordinates. All atoms were allowed to move in the GGA+U

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... ations because of convergence problems caused by relaxation-induced charge localization at the bottom surface. Structures were relaxed using the BFGS algorithm until forces were below 10 -3 au (∼0.05 eV/Å). Due to the large size of the simulation cell (10.26 Å x