  We study the role of electron correlations in the formation of the surface
charge-density-wave state in the Sn/Ge(111) interface. The Fermi energy of the
overlayer is treated as a dynamical variable, which undergoes a substantial
renormalization by the interaction. We show that the Fermi level turns out to
be pinned to a Van Hove singularity in the density of states, which explains
the formation of the charge-density-wave, the observation of a very flat band
in photoemission experiments and the reduction of the spectral weight in the
low-temperature phase.
