We have studied dissociative H-2 adsorption on the H/Si(100) surface by means of pseudopotential density functional theory calculations. This work is directly motivated by the observations of Biederman et al. [Phys. Rev. Lett., 83, 1810 (1999)]: "H-2 adsorption is significantly promoted in an interdimer configuration of two adjacent singly occupied dimers." We find that there is no adsorption barrier on the local site where two quasi-free dangling bonds are available; the adsorption energy is estimated to be 2.6 eV per H-2 within the local density approximation. We present maximally localized Wannier functions that clearly illustrate the behavior of dangling orbitals upon the approach of H-2. The results suggest that intradimer pi -bonding disruption is crucial for the significant promotion of H-2 adsorption on Si(100).