Interatomic potential models used for atomistic simulations of insulating oxides were reconsidered for the case of rutile. The cohesive energy of oxides involved an electrostatic part and a short-range part whose relative importance differed from model to model. The electrostatic part could be evaluated by considering either fixed point atomic charges or charges that were allowed to vary in response to the local atomic environment; with a shielding correction to Coulombic interactions at short range. The latter approach was analyzed within the framework of the Rappé and Goddard charge equilibration scheme. It was concluded that it was an efficient model for describing heterogeneous situations due to point defects or surfaces. Whatever the description of the electrostatic part of the energy, several short-range interatomic potentials were found to describe properly crystal bulk properties such as cohesive energy and elastic constants. In order to compare the efficiency of various short-range potentials, rutile was selected; for which experimental values of the formation energy of an O vacancy were available. By combining these with the cohesive energy, it was possible to analyze accurately the energetics of TiO2 as a function of the potentials. It was shown firstly that the Morse potential was not adapted to O-O interactions, and that pair-wise potentials between Ti-O pairs were not suitable for describing defects. As a result, a model was proposed that combined the Rappé and Goddard description for electrostatic energy, a Buckingham potential for O-O interactions and an N-body potential for the covalent part of the Ti-O interactions. This model was tested for the bulk, O vacancy and surfaces of rutile, and provided results which fitted experimental data very satisfactorily.

Use of a Variable-Charge Interatomic Potential for Atomistic Simulations of Bulk, Oxygen Vacancies and Surfaces of Rutile TiO2. Hallil, A., Tétot, R., Berthier, F., Braems, I., Creuze, J.: Physical Review B, 2006, 73[16], 165406