  We propose a new structural model for the Si(111)-(5x2)-Au reconstruction.
The model incorporates a new experimental value of 0.6 monolayer for the
coverage of gold atoms, equivalent to six gold atoms per 5x2 cell. Five main
theoretical results, obtained from first-principles total-energy calculations,
support the model. (1) In the presence of silicon adatoms the periodicity of
the gold rows spontaneously doubles, in agreement with experiment. (2) The
dependence of the surface energy on the adatom coverage indicates that a
uniformly covered phase is unstable and will phase-separate into empty and
covered regions, as observed experimentally. (3) Theoretical scanning tunneling
microscopy images are in excellent agreement with experiment. (4) The
calculated band structure is consistent with angle-resolved photoemission
spectra; analysis of their correspondence allows the straightforward assignment
of observed surface states to specific atoms. (5) The calculated activation
barrier for diffusion of silicon adatoms along the row direction is in
excellent agreement with the experimentally measured barrier.
