The properties of ammonia adsorbed at flat, step and kink sites on a MgO(100) surface have been investigated using density functional theory. A finite MgO cluster is used to represent the MgO(100) surface. Bonding is primarily determined by electrostatic interactions, which can have long range effects on these ionic surfaces. To account for such effects, cluster models of various sizes have been compared and the effects of surrounding the cluster by an array of fixed point charges have been explored. Several different structures have been found for ammonia bound at the various surface sites. The calculated adsorption energy on a flat surface is 19-21 kcal/mol, which is in reasonable agreement with experimental estimates of 13-17 kcal/mol at low coverage. Binding at kink sites is strongly favored over binding on the flat surface and dissociation is feasible only at kink sites. The electrostatic environment has a much greater effect on adsorption at step and kink sites than on the flat surface. The reaction path for diffusion of ammonia on a flat MgO(100) surface is explored. The nature of the ammonia-MgO interactions, the effects of the electrostatic environment, and considerations in the choice of cluster models are discussed.