  We have used Low Energy Electron Diffraction (LEED) I-V analysis and ab
initio calculations to quantitatively determine the honeycomb chain model
structure for the Si(111)-3x2-Sm surface. This structure and a similar 3x1
recontruction have been observed for many Alkali-Earth and Rare-Earth metals on
the Si(111) surface. Our ab initio calculations show that there are two almost
degenerate sites for the Sm atom in the unit cell and the LEED I-V analysis
reveals that an admixture of the two in a ratio that slightly favours the site
with the lower energy is the best match to experiment. We show that the I-V
curves are insensitive to the presence of the Sm atom and that this results in
a very low intensity for the half order spots which might explain the
appearance of a 3x1 LEED pattern produced by all of the structures with a 3x2
unit cell.
