Adsorption of elemental mercury (Hg) on small neutral, cationic and anionic gold clusters (Au-n, n = 1-6) has been studied by using the density functional theory (DFT). Results of this investigation show that frontier molecular orbital theory is a useful tool to predict the selectivity of Hg adsorption. It is found that adsorption of Hg on neutral, cationic and anionic Au-n (n = 1-6) clusters are thermodynamically favorable. The binding energies of Hg on the cationic Au-n clusters are greater than those on the neutral and anionic clusters. Natural bond orbital (NBO) analysis indicates that the flow of electrons in the neutral and charged clusters is mainly due to the s orbitals of Hg and Au. Results of NBO analysis also indicate that the binding energy of Hg with Au-n clusters is directly proportional to the charge transfer, i.e. greater is the charge transfer, higher is the binding energy. (C) 2012 Elsevier Ltd. All rights reserved.