State mixing for quasiparticles at surfaces: Nonperturbative GW approximation

We present ab initio calculations for both the wave functions and the energ ies of single quasiparticles. The conventional quasiparticle approach evalu ates energy-level corrections to first order in the difference between the self-energy and the Kohn-Sham exchange-correlation potential. Here,we also recalculate the quasiparticle states. At the example of the GaAs(110) surfa ce we show that this nonperturbative treatment is important far surfaces wi th electronic states close in energy but different with respect to their lo calization. As a sensitive observable the reflectance anisotropy is studied . [S0163-1829(99)15947-2].