The adsorption characteristics and change in work function of Cs on a (2 x 2) GaN(0 0 0 1) surface with a coverage from 1/4 to 1 monolayer (ML) have been investigated using density functional theory with a plane-wave ultrasoft pseudopotential method based on first-principles calculations. The results show that the most stable positions of Cs adatoms on GaN(0 0 0 1) surfaces are at N-bridge and H3 sites for 1/4 ML coverage. As the Cs atomic coverage is increased, adsorption energy and stability reduce and achieve saturation when the Cs adatom coverage is 3/4 ML. The transfer of Cs6s electrons to Ga atoms in the outermost layer decreases the work function of the system. (C) 2012 Elsevier B.V. All rights reserved.