The tri-down and tetra-down structures were found to have lower energies compared to the tri-up and tetra-up structures, respectively. This is somewhat expected based on our knowledge that PtH bonds are stronger than PtO bonds. Therefore, we considered the tridown (Figure ) and tetra-down (Figure )) structures for dissociation barrier calculations. In both the cases, one of the the molecules directed towards the metal surface dissociates. It is found that after dissociation, the intact O-H bond remains parallel to the surface while the dissociated H atom gets adsorbed on a nearby Pt site. Since the metal surface provides catalytically active sites, it is only reasonable that one of the molecules directed towards the surface dissociates. The initial water structures and their corresponding transition states, and the final dissociated structures are shown in the insets of Figure . In the free structure, the activation barrier is found to be 0.72 eV. For the trihedral structure, the value is slightly higher and is 0.88 eV. On the other hand, the activation barrier for the tetrahedral structure turns out to be 1.71 eV, much higher than the other two structures. The MEP of the Volmer reaction for three different water structures are shown in Figure . These results clearly indicate that a much larger kinetic barrier has to be overcome for dissociation of an O-H bond in tetrahedrally coordinated water compared to free and trihedral water. This is likely due to the more compact network of hydrogen bonds in the former. The activation energy of water dissociation indeed increases in the order free water < trihedrally coordinated water < tetrahedrally coordinated water, leading to a reverse order of their dissociation rate, and hence the rate of the Volmer step. Thus, we can convincingly ascribe the observed slowdown of the HER rate with increasing alkali concentration to the increase in population of tetrahedrally coordinated water that has the largest dissociation barrier. The dissociated H atom is represented by blue sphere.