We have carried out ab initio simulations on the ambient pressure phase I of solid ammonia, and on the high-pressure phase IV. Our plane-wave pseudopotential calculations yield very good agreement with existing structural data, lattice energies, and equations of state. We have also studied the tendency toward symmetrization of the hydrogen bonds at high pressures and find that, unlike pure ice, this process should not occur at experimentally achievable pressures, i.e., <300 GPa. Moreover, our results show that ammonia IV does not contain a bifurcated hydrogen bond, as has previously been suggested. (C) 2003 American Institute of Physics.