Relaxation of (111) silicon surface atoms from studies of Si₄H₉ clusters

Self-consistent Hartree-Fock and generalized valence bond calculations have been performed on clusters modeling the (111) silicon surface. We find that the surface state is accurately described as a dangling bond surface orbital with 93% p character. We determined the optimum relaxation of the surface layer to be 0.08 Å toward the second layer. In the positive ion, the surface atom relaxes toward the second layer by an additional 0.30 Å and for the negative ion the surface atom moves toward the vacuum 0.25 Å. The vertical ionization potential was found to be 5.78 eV (experimental values are 5.6 - 5.9 eV) while the calculated adiabatic electron affinity is 3.02 eV.