SELF-ENERGY EFFECTS ON THE SURFACE-STATE ENERGIES OF H-SI(111)1X1

We have calculated the quasiparticle energy of the occupied surface st ates of the H-Si(111)1x1 surface. The electron self-energy operator is expanded to first order in the screened Coulomb interaction in the GW approximation. The results explain the data from recent high-resoluti on angle-resolved photoemission spectroscopy. Comparison of the quasip article surface-state energies with those from local-density-functiona l eigenvalues shows that the self-energy corrections are very large, t ypically two to three times larger than the corrections found in previ ous calculations on other semiconductor surface systems. We have also performed a frozen-phonon study of the stretching mode of the Si-H bon d. As observed in several recent experiments and theoretical studies, a large anharmonicity is found.