We predict the structures and detailed energetics for the dissociative adsorption of NH3 to form NH2 and H adsorbed on a single Si dimer on the Si(100)-2 x 1 surface at the MRSDCI (multireference single and double excitation configuration interaction) level of theory. We predict that this dissociation involves two steps : (i) barrierless molecular chemisorption of NH3 followed by (ii) activated N-H bond cleavage of NH;,,, to form NH2(a) + H-(a). While the second step involves a barrier, its relatively small height renders the overall reaction barrierless. The extremely high adsorption exothermicity (similar to 75 kcal/mol) results in a very high desorption barrier. These results can explain the experimentally determined high sticking probability of NH3, the observation of NH3(a), at low temperatures, and the observed stability of NH2(a) and H-(a) on the Si(100) surface up to similar to 600 K. Additionally, our CASSCF level (complete active space self-consistent-field) calculated geometries for the dissociatively adsorbed species agree with structures proposed to explain experimental data.