The Influence of Gradient Corrections on Bulk and Surface Properties of TiO2 and SnO2

First-principles calculations based on density functional theory and the pseudo\-potential method have been used to investigate the influence of gradient corrections to the standard LDA technique on the equilibrium structure and energetics of rutile TiO$_2$ and SnO$_2$ perfect crystals and their (110) surfaces. We find that gradient corrections increase the calculated lattice parameters by roughly 3~\%, as has been found for other types of material. Gradient corrections give only very minor changes to the equilibrium surface structure, but reduce the surface energies by about 30~\%.