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3e-Fig. 3h). Nine sharp peaks are illustrated in the PDOS projected on the O2 atom in the isolated HSCH$_{2}$COOH molecule (Fig. 2b). Upon the HSCH$_{2}$COOH is deposited on the gold surface, some peaks are suppressed (Fig. 3e), however, the PDOS projected on the O2 atom is insensitive to the adsorption and the dissociation of the mercaptan-hydrogen atom (Fig. 3e, Fig. 3f). Fig. 3g and Fig. 3h show that some peaks in Fig. 2b disappear and some others disperse after the carboxylic acid group is deprotonated, which indicates that the deprotonation changes the electronic states around the O2 atom. 

To further elucidate the interacting bond between the HSCH$_{2}$COOH, SCH$_{2}$COOH, HSCH$_{2}$COO − , SCH$_{2}$COO − molecule and the Au(111) substrate, we calculate the charge- density difference: Δ ρ ( ⃗ r ) = ρ$_{ads/sub}$ ( ⃗ r ) − ρ$_{sub}$ ( ⃗ r ) − ρ$_{ads}$ ( ⃗ r ), where ρ$_{ads/sub}$ , ρ$_{sub}$ , and ρ$_{ads}$ are the electron charge densities of the relaxed adsorbate-substrate system, of the clean relaxed surface, and of the isolated but adsorptionlike deformed adsorbate (without substrate), re- spectively. The isodensity surfaces of the charge-density difference for the structures of the HSCH$_{2}$COOH, SCH$_{2}$COOH, HSCH$_{2}$COO − , and SCH$_{2}$COO − on the Au(111) substrate are depicted in Figs. 4a-4d. In Fig. 4, we display only the surrounding part of the S-Au bond. As we know, two p-electrons of the sulfur in the HSCH$_{2}$COOH molecule form a lone pair, and the region around the top of the gold is a charge depletion area $^{30}$. In the configuration 

