Relaxation processes due to two opposite surfaces in a slab of hydroxyapatite with two (100) free surfaces are reported. Interatomic potentials for hydroxyapatite, chloroapatite and fluoroapatite were refined using elastic and vibrational data. Systems with two free (100) surfaces were relaxed. It was found that the relaxation processes which occurred at the surface included the polarization of layers close to the surface, distortion of the channels in which the hydroxide ions sit and distortion of the tetrahedral phosphate ions. Some of these relaxations only occur in the unit cells closest to the surface; others persist for a few unit cells distance into the bulk. The implications of these results for the growth morphology of hydroxyapatite in bones are discussed.