The (root 3 x root 3)R30 degrees-Cu2Si/Cu(1 1 1) surface alloy that forms during high temperature dosing of silane (SiH4) on Cu(1 1 1) has been investigated using LCAO-DFT. Simulated STM images have shown that experimental images may be interpreted as a mixed phase system consisting of Si ion cores bound in HCP, FCC and twofold bridge sites with a ratio of 25:25:50 rather than previously proposed models where the Si ion cores were bound in only FCC and HCP sites. The new model is shown to be consistent with previously published NIXSW studies. (c) 2009 Elsevier B.V. All rights reserved.