Despite the fact that attempts were made to describe the interaction of a single H2O molecule with a mercury surface, using both semi-empirical and ab initio quantum chemical calculations, reliable microscopic information on the HgH2O interface is still lacking. In this work, non-empirical quantum chemical calculations were carried out to study water molecule adsorption on an uncharged mercury electrode. The mercury surface was modelled by a cluster Hg(n), with n = 6, 7. The effect of electronic correlation plays an important role. An "on-top" position of the H2O molecule with the dipole moment pointing away from the surface reveals an adsorption energy minimum (DELTAE(ads)) of -38.5 kJ mol-1. The dipole reorientation energy was estimated to be 21.8 kJ mol-1. According to our results, the dependence of DELTAE(ads) on the tilt angle has no limit. Analysis of the chemical binding between the cluster and the H2O molecule shows the electrostatic nature of the binding. The mean field approximation was applied to describe the interaction between the adsorbed H2O molecules in a monolayer. The results were in agreement with experimental data.