The extent of relaxation of the α-Al2O3 (0001) surface and the reliability of empirical potentials

The geometrical structure of the polar, Al-terminated surface of corundum has been investigated by different theoretical methods ranging from empirical pair potentials to periodic Hartree–Fock or density functional theory calculations. The different methods agree in predicting a structure for the bulk, which is close to the experimental one, and an inward relaxation of more than 50% in agreement with experiment. However, a rather strong disagreement exists concerning the quantitative amount of relaxation of the surface layers. Accurate density functional theory calculations for slab models considering up to 18 layers with 30 atoms in the unit cell strongly suggest that the degree of relaxation of the outermost Al atomic layer is much larger than the values obtained by other methods. Reasons for the different description are given and implications for the use of pair potentials discussed.