The interaction of atomic fluorine and chlorine with the Si(111) surface has been studied by using clusters of Si atoms to stimulate the substrate. The largest cluster contains ten Si atoms, representing the first four layers of the Si surface. An F or Cl atom is added to three high-symmetry adsorption sites. In the on-top site, the halogen adatom is directly above a surface Si atom. The open and eclipsed sites are both threefold with the adatom at the center of a triangle of first-layer atoms. In the open site, the adatom is directly above a fourth-layer Si while in the eclipsed site it is above a second-layer Si. Ab initio Hartree-Fock wave functions have been calculated, and the energy of the system is studied as a function of the vertical distance of the adatom from the surface (first) Si layer. Equilibrium bond distances, vibrational energies of the adatoms, and binding energies for adsorption are calculated. Bonding mechanisms and properties for the different sites are compared. For the on-top site, the binding energy De is 3.2 eV (1.6 eV) for F (Cl), and the equilibrium distance from the surface re is 1.7 (2.2). For F at the open site, two attractive minima are found: Re=1.3 and -1.4; De is found to be 0.5 eV for both positions, the barrier height for penetration is 1 eV. For Cl at this open site, De is 0.4 eV and re is 1.9. The barrier height is 13 eV. At the eclipsed site, De is 0.5 eV (0.2 eV) for F (Cl) with re being 1.7 (2.2). The calculated De's for Cl indicate that the most stable chemisorption site for Cl on Si(111) is the onefold on-top site. The vibrational energies for the motion of Cl normal to the surface are found to be substantially different for sites with different surface coordination and should be experimentally accessible in electron-energy-loss spectroscopy. The difference of calculated core-ionization energies for F at the on-top site and at the open site are compared with observed x-ray photoelectron spectroscopy data. This comparison suggests the formation of an intermediate-surface species, where the F atom has penetrated into the lattice, before the final product, volatile SiF4, of the reaction between F and Si is formed. All the results are in agreement with the observed different reactivity of F and Cl with Si. They provide a model for understanding reactions relevant in plasma etching. © 1983 The American Physical Society.