The interaction of a hydrogen atom with the Si(111) surface is studied using clusters with up to ten silicon atoms in order to model the threefold open site. On the basis of ab initio Hartree-Fock linear-combination-of-atomic-orbital theory the potential curve is calculated as a function of distance of the hydrogen atom from the surface. Using a two-configuration multiconfigurational self-consistent-field (MCSCF) wave function, which allows for both ionic and covalent structures, leads not only to the proper dissociation behavior at infinite separation but also shifts down the SCF ground-state energy curve by about 1 eV. This effect leads to a well with a minimum at 1.4 above the surface and a depth of about 1 eV. The SCF barrier height for penetration of the surface of 2.7 eV is reduced to 1.8 eV. We compute a vibrational frequency for motion normal to the surface of about 100 meV, considerably smaller than the vibrational energy for the head-on adsorption site. The study shows that SCF cluster calculations of this type give qualitatively correct potential curves with minima and barriers at about the right distances but binding energy errors of about 1 eV due to the neglect of the main correlation contributions. The possible role of the interaction of hydrogen at the open site in the formation of the trihydride phase is discussed. © 1981 The American Physical Society.