TABLE III: The geometries and adsorption energies for the structures of the HSCH$_{2}$COO − molecule on the Au(111) surface at 0.25ML. initial θ d$_{S}$$_{−}$$_{Au}$ optimized θ tilt d$_{S}$$_{−}$$_{Au}$ E$_{ads}$ site ˚ A site deg direct ˚ A eV bri 0 2.60 bri 1.1 hcp 2.86 0.44 45 2.60 bri 50.6 hcp 2.66 0.67 90 2.60 bri 82.8 hcp 2.99 0.96 fcc 0 2.60 bri-fcc 8.0 fcc 2.67 0.54 45 2.60 bri-fcc 65.4 fcc 2.74 0.92 90 2.60 top-fcc 77.1 hcp 3.38 0.95 hcp 0 2.60 hcp 1.5 fcc 2.75 0.66 45 2.60 top-hcp 46.6 fcc 2.57 0.81 90 2.60 bri-hcp 78.0 fcc 3.62 0.83 top 0 2.60 top 5.7 hcp 2.85 0.30 45 2.60 top 56.6 hcp 2.65 0.94 90 2.60 top 82.9 hcp 2.60 1 . 13 HSCH$_{2}$COO − molecule prefers to stay on the atop site of the gold atom, as indicated in Fig. 1c. The corresponding adsorption energy is 1.13 eV which is larger than that of the thio- glycolic acid on the Au(111) surface. The S-Au bond length is around 2.60 ˚ A and the angle θ is 82.9 ◦ , which means the HSCH$_{2}$COO − molecule is lying down on the gold substrate. If the mercaptan-H atom in the HSCH$_{2}$COO − is detached from the sulfur atom, the resulting compound is the SCH$_{2}$COO − . Table IV shows that the sulfur atom favors the fcc hollow center with the adsorption energy 2.34 eV. The polar angle between the normal vector of the gold surface and the S-C2 direction is 53.4 ◦ ; so, after losing mercaptan-H atom, the SCH$_{2}$COO − molecules begin to rise, see Fig. 1d. Note that there is a # marked configuration in Table IV, whose adsorption energy is 2.33 eV - very closed to 2.34 eV. The 8 D. The SCH$_{2}$COO − molecule on the Au(111) surface