We report first principles calculations to analyze the ruthenium adsorption and diffusion on GaN(0 0 0 1) surface in a 2 x 2 geometry. The calculations were performed using the generalized gradient approximation (GGA) with ultrasoft pseudopotential within the density functional theory (DFT). The surface is modeled using the repeated slabs approach. To study the most favorable ruthenium adsorption model we considered T-1, T-4 and H-3 special sites. We find that the most energetically favorable structure corresponds to the Ru- T-4 model or the ruthenium adatom located at the T-4 site, while the ruthenium adsorption on top of a gallium atom (T-1 position) is totally unfavorable. The ruthenium diffusion on surface shows an energy barrier of 0.612 eV. The resultant reconstruction of the ruthenium adsorption on GaN(0 0 0 1)- 2 x 2 surface presents a lateral relaxation of some hundredth of angstrom in the most stable site. The comparison of the density of states and band structure of the GaN(0 0 0 1) surface without ruthenium adatom and with ruthenium adatom is analyzed in detail. (c) 2008 Elsevier B.V. All rights reserved.