We have performed first-principles calculations of energetic stability and electronic structure of nickel adsorption and incorporation on a 2 x 2 reconstructed GaN(0 0 0 1) surface. Our total energy results show that the most stable positions of a Ni adatom on the reconstructed 2 x 2-T-4 GaN(0 0 0 1) surface are at the H-3(a0) and T-4 adsorption sites. We found that the Ni adatom diffusion energy barrier between the H-3(a0) and T-4 sites is around 0.16eV, which is an indication of a significant Ni adatom diffusion on this surface. In addition, calculating the relative surface energy of several configurations, we constructed a phase diagram showing the energetically most stable surfaces as a function of the Ga chemical potential. Based on these results, we found that Ni adsorption is energetically more favorable compared with Ni incorporation in the Ga-substitutional and interstitial sites. In addition, confirming previous experimental results, we found that the growth of NiGax and Ni monolayers on the GaN(0 0 0 1) surface is possible. (c) 2012 Elsevier B.V. All rights reserved.