Slab and cluster models are used to study Hz desorption from a single dimer of the Si(100)-2 x 1 surface. The cluster models are constructed using geometries obtained from slab-model optimizations. The largest cluster model considered, Si89H62, 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 Hz desorption, calculated using the Si89H62 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.