The intradimer pre-paired desorption mechanism for hydrogen on Si(100)-(2 x 1) has been studied with density functional calculations using 1-dimer and 3-dimer cluster models of the surface. We find that adsorption/desorption occurs in a two-step process through a metastable dihydridelike intermediate, Two transition states are identified in this pathway. We confirm that the transition states are at saddle points by computing the vibrational frequencies, and that the reaction path from the monohydride to the desorbed state goes through the intermediate by performing. eigenvector-following calculations from the two transition states. The effects of clutter size-and basis set on the energetics are investigated. It is observed that energetics from 3-dimer cluster B3LYP6-311G** calculations are in reasonable agreement with experimental data for both the adsorption and desorption barriers. We find an adsorption barrier of 0.65 eV and a desorption barrier of 2.94 eV: We obtain a distance of approximately 3.21 Angstrom between the silicon dimer atoms in the intermediate structure, implying a broken dimer bond, which we confirm by considering the electron density plots for the structure. Thus our results suggest that during each adsorption or desorption event the dimer bond is broken and reformed. The forces acting on the silicon dimer atoms and the hydrogen atoms along the reaction path are also investigated and these provide a picture of the coupling of the adsorption/desorption process to surface vibrations, particularly to the dimer bond breaking and reforming. This occurrence of dimer bond breaking and reforming along the reaction _ path suggests an explanation for the experimentally observed large surface-temperature activation of the adsorption process.