The bonding characterization of Br on the Si(100) surface has been studied by the atom superposition and electron delocalization molecular orbital theory, using 2*1 symmetric- and buckled-dimer models for this surface. From the total energy minima, the authors have found two stable adsorption sites for Br bonding in the symmetric-dimer model. One is along the surface dangling bond with a Br-Si bond length of 2.16 AA and a bond angle of 19 degrees to the surface normal; the other is the bridge site with a Br-Si bond length of 2.13 AA. In the buckled-dimer model, Br is predicted to adsorb onto the dangling bonds and further to enhance the buckling. The calculated bond lengths and angles are in good agreement with the recent X-ray standing-wave (XSW) experiment. Moreover, the authors give a new explanation for the low coherent fraction f_c in the XSW study.