We have provided an understandings at atomic level of the Hg0 FLAG tag Peptide on Agn/TiO2 (n = 2, 4, and 8) composites by using the DFT calculations. Firstly, the optimized geometries of small Agn clusters have been explored. Then Agn clusters are deposited over the rutile TiO2 (1 1 0) surface and were optimized for finding their local minima. After that, the Hg0 adsorptions over (i) the isolated Agn clusters, (ii) the Agn/TiO2 composites, and (iii) the bare TiO2 surface were examined. From the results, adsorptions of the Hg0 over the Agn/TiO2 models are relatively stronger than those in both isolated Agn clusters and the bare TiO2, which can explain the experimental observations well. We found that the Hg0 acts as electron donor once it is adsorbed on the studied sorbents. For that reason, the strong interaction of the Hg0 atom and the Ag/TiO2 composite is an influence of electron transfer from the Hg0 to the composites. Greater electron transfer from the Hg0 promotes stronger Hg adsorption on the sorbent. In addition, we found that the Hg0 adsorption does not significantly depend on the Ag cluster size (in range of 2–8 atoms-clusters) but does depend strongly on the value of d-band center of the silver composites and amount of charge transferred from the Hg0.