  Density Functional Theory calculations are used to investigate the role of
substrate-induced cooperative effects on the adsorption of water on a partially
oxidized transition metal surface, O(2x2)/Ru(0001). Focussing particularly on
the dimer configuration, we analyze the different contributions to its binding
energy. A significant reinforcement of the intermolecular hydrogen-bond
(H-bond), also supported by the observed frequency shifts of the vibration
modes, is attributed to the polarization of the donor molecule when bonded to
the Ru atoms in the substrate. This result is further confirmed by our
calculations for a water dimer interacting with a small Ru cluster, which
clearly show that the observed effect does not depend critically on fine
structural details and/or the presence of co-adsorbates. Interestingly, the
cooperative reinforcement of the H-bond is suppressed when the acceptor
molecule, instead of the donor, is bonded to the surface. This simple
observation can be used to rationalize the relative stability of different
condensed structures of water on metallic substrates.
