Concerted Use of Slab and Cluster Models in an ab Initio Study of Hydrogen Desorption from the Si(100) Surface

Slab and cluster models are used to study H<INF>2</INF> desorption from a single dimer of the Si(100)−2 × 1 surface. The cluster models are constructed using geometries obtained from slab-model optimizations. The largest cluster model considered, Si<INF>89</INF>H<INF>62</INF>, contains eight surface dimers and gives reaction and activation energies for desorption nearly identical with the slab-model values when the same electronic structure method is used. The barrier for H<INF>2</INF> desorption, calculated using the Si<INF>89</INF>H<INF>62</INF> cluster model and the Becke3LYP functional, is 64.3 kcal/mol. When this result is corrected for the effects of basis set expansion and vibrational zero-point energy correction, the barrier decreases to about 61.0 kcal/mol, which is only 4.0 kcal/mol greater than the observed desorption barrier.