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 × 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 NH3(a) 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 (∼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 ∼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.

Curiously, H 2 desorption from Si͑100͒-2ϫ1 follows approximately first-order kinetics rather than the expected second-order kinetics, arousing interest about the mechanism involved in the desorption process. We investigate the energetics and rate constants of three proposed mechanisms for H 2 desorption from Si͑100͒-2ϫ1, namely, the prepairing mechanism, the isomerization mechanism, and the isolated dihydride mechanism, using complete active space self-consistent-field and multireference single-and double-excitation configuration-interaction calculations. We find the desorption barrier for the isolated dihydride mechanism to be 2.49 eV, the only barrier in excellent agreement with the experimentally determined barrier ͑ϳ2.5 eV͒. The isolated dihydride mechanism also provides the only calculated desorption rate constant close to experimental values. Finally, we show that this mechanism is able to explain the experimentally observed apparent violation of detailed balance of H 2 adsorption/desorption on Si͑100͒, as well as other experimentally observed dynamics. ͓S0163-1829͑96͒07939-8͔

We review the young field of ab initio molecular dynamics applied to molecule-surface reactions. The techniques of ab initio molecular dynamics include methods that use an analytic potential energy function fit to ab initio data and those that are fully ab initio. In this review, we focus on the insights provided by ab initio-based molecular dynamics that are currently unavailable from experimental studies and discuss current techniques and limitations. As an example of how different aspects of a problem can be tackled with state-of-the-art theoretical tools, we consider the well-studied case of H2 desorption and adsorption from the Si(100)-2 x 1 surface.

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