The energetics involved in the chemisorption of NH3 on Si(100)-(2 x 1) have been examined using ab initio and nonlocal density functional theory. One- and two-dimer cluster models were employed to model the Si(100)-(2 x 1) surface. By using various exchange-correlation functionals and Gaussian split-valence basis sets, we have obtained the geometries of the molecularly adsorbed and dissociatively chemisorbed states, as well as the hitherto unreported geometry of the transition state which exists between the two states. The geometries of the various states have been rationalized based on either electrostatic or orbital interactions. In addition, calculations were also performed on models which contain a second-adlayer ammonia molecule to yield several possible geometries for the extrinsic precursor state. The extrinsic precursor ammonia binding energies for the various geometries found are in the range of 3.93-8.80 kcal/mol. The energetics of the chemisorption process and the binding energies of the extrinsic precursor ammonia are in good agreement with available experimental data.