The surface energetics of hydroxyapatite was studied using density functional theory within the generalized gradient approximation. The effects of slab thickness, vacuum width between slabs and surface relaxation on surface energy have been tested. The results show that as long as the vacuum width is 12 Angstrom or more, the influence of vacuum width on surface energy is small. Slabs with a thickness of 15.6 Angstrom are enough to obtain meaningful results. Relaxation of the slab surfaces has an apparent effect on surface energy. Surface relaxation modifies the relative order of the morphological importance compared with unrelaxation. (C) 2004 Elsevier B.V. All rights reserved.