TABLE IV: The geometries and adsorption energies for the structures of the SCH$_{2}$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-hcp 9.2 fcc 2.51 2.16 45 2.60 bri 44.9 hcp 2.45 2.01 90 2.60 bri-fcc 78.6 hcp 2.43 2.21 fcc 0 2.60 fcc 7.8 hcp 2.47 2.14 45 2.60 bri-fcc 53.4 hcp 2.49 2 . 34 90 2.60 bri-fcc 83.2 hcp 2.49 2.28 hcp 0 2.60 hcp 0.5 fcc 2.49 2.11 45 2.60 bri-hcp 39.8 fcc 2.44 2.04 90 2.60 bri-hcp 76.4 fcc 2.45 2.27 top 0 2.60 bri-top 10.1 bri 2.54 1.88 #45 2.60 bri-hcp 51.8 fcc 2.43 2.33 90 2.60 top 81.8 fcc 2.38 2.08 angle θ for top-45 ◦ structure (the # marked configuration) is 51.8 ◦ . The stable structures in Table IV indicate that when the SCH$_{2}$COO − is deposited on the Au(111) surface, the molecule rises up to a certain level 8 (see Fig. 1d). To understand how the dissociation of the mercaptan-hydrogen atom and the deproto- nation of the carboxylic acid group play roles in the adsorption process, we calculate the partial density of states (PDOS) projected on the S and O2 atoms in the HSCH$_{2}$COOH, SCH$_{2}$COOH, HSCH$_{2}$COO − , SCH$_{2}$COO − molecule deposited on the Au(111) substrate. There are three sharp peaks in the PDOS projected on the S atom in the isolated HSCH$_{2}$COOH molecule (Fig. 2a). The major contributions of three peaks come from π , σ and π ∗ orbitals in the S-C bond. To calibrate the Fermi level for the isolated molecule, E. Electronic Structures 9