In order to calculate the dissociation energy of water, we explicitly arranged water molecule(s) in different patterns above the Pt (111) surface to model the three different coordination environments as shown in Figure . For free water structure, we put one water molecule above the Pt surface corresponding to a coverage of 1/9 (Figure ). In this case, there are no other water molecules in the surrounding, and thus no hydrogen bonds, giving the free water structure. For trihedral coordination, we put one layer of hexagonal water (6 water molecules) above the Pt surface at a coverage of 2/3. Ogaswaraet. al. have observed experimentally that the water structure above the Pt (111) surface forms a hexagonal ice-like pattern. Previous DFT studies also reported that the geometry of water molecules above the metal surface is based on crystalline ice structure arranged in hexagonal pattern. As shown in the Figures and, alternate water molecules (3 out of 6) have both O-H bonds oriented parallel to the metal surface. The remaining water molecules have one O-H bond parallel to the surface. The other O-H bond can be either vertically up or down towards the surface. Accordingly, these two structures are named as trihedral-up (tri-up) and trihedral down (tri-down) structures. In both these structures, each water molecule forms three hydrogen bonds with the neighboring ones. For tetrahedral arrangement, we have added a second hexagonal ice-like layer of water molecules above the trihedral arrangement. As a result, each water molecule now makes four hydrogen bonds, giving rise to tetrahedral coordination (Figures and). As in the trihedral structure, there are also two different configurations for the tetrahedral structure. These are named as tetrahedral-up (tetra-up) and tetrahedral-down (tetra-down) based on the orientation (up or down) of the O-H bonds in the water molecules in the first layer above the surface.