The chemisorption of methanethiolate on Ni(111) is treated using a many-electron embedding theory, modeling the lattice as a 28-atom, three-layer cluster with the Ni atoms fixed at the positions from the bulk. Methanethiolate binds strongly to the Ni(111) surface at both high and low symmetry sites. Calculated adsorption energies are 61 kcal/mol for an equilibrium sulfur-surface distance of 1.99 Angstrom at three-fold sites, 61 kcal/mol with a sulfur-surface distance of 2.06 Angstrom at bridge sites, and 57 kcal/mol with a sulfur-surface distance of 2.13 Angstrom at atop sites. Different equilibrium geometries are predicted for CH3S at three-fold, bridge, and atop sites. The methanethiolate C-S axis is nearly perpendicular to the surface, inclined only about 5 degrees from the surface normal, at the three-fold site. The C-S axis is found to be tilted away from the surface normal by 45 degrees at the bridge site, and by 55 degrees at the atop site. For the C-S axis perpendicular to the surface, the calculated C-S stretching vibration is 730 cm(-1), and the CH3S-surface stretch is 281 cm(-1) at the three-fold site. For a 45 degrees tilt at the bridge site and a 55 degrees tilt at the atop site, the CS stretching vibrations are 677 and 698 cm(-1), respectively.