The results of atomistic calculations based on an ab initio tight-binding method are reported in this work for four configurations of the perfect (a) over right arrow -screw basal dislocation in wurtzite GaN: pure shuffle, pure glide and two mixed shuffle-glide. Configurations with pure character (shuffle or glide) are found to be stable, whereas those with mixed shuffle-glide character are found to be transition-like configurations. Further, the calculations predict that the pure glide configuration, containing threefold coordinated atoms with an sp(2) hybridization, to be the most energetically favourable. (c) 2006 Elsevier B.V. All rights reserved.