Using first principles total energy calculations we have investigated the initial stages of the adsorption of Sc and ScN thin films on GaN(0 0 0 1) surfaces under both N and Ga rich conditions. In an ideally GaN(0 0 0 1) bulk terminated surface, and when the Sc atom is constrained to remain on top of the surface, the T4 site configuration is the most favorable. However a structure in which the Sc atom replaces a Ga atom of the first monolayer and the displaced Ga atom occupies a T4 site (forming bonds with Ga atoms only) has the lowest energy. Results are similar for Ga rich conditions: if the Sc atom is constrained on top of the surface, it occupies the T4 site. However, if it is allowed, it will occupy sites in the third (from top) Ga layer and it will form ScN. For a full monolayer of Sc atoms, three different configurations are possible, in all of them there is formation of scandium nitride: a ScN bilayer terminated configuration for N rich conditions, a ScN bilayer underneath a Ga bilayer for Ga rich conditions, and a ScN bilayer under a Ga layer for intermediate configurations. In all three geometries, the ScN are in wurtzite like configurations with distorted bond angles and the surfaces are metallic. (C) 2012 Elsevier B. V. All rights reserved.