Aspects of the percolation process for hydrogen-bond networks in water

Sets of configurations selected from three molecular dynamics simulations for liquid water have been analyzed for the distribution of hydrogen-bond clusters. Two simulations correspond to water at 1 g cm−3, while the third corresponds to highly compressed water at 1.346 g cm−3. An energy criterion was adopted for existence of a hydrogen-bond between two molecules. As the cutoff value for bonding increases (becomes more permissive), a bond percolation threshold is encountered at which initially disconnected clusters suddenly produce a large space-filling random network. At least for the model studied, any chemically reasonable definition of ’’hydrogen-bond’’ leads to this globally connected structure though a few disconnected fragments inhabit its interior. Although some polygonal closures can exist, the critical percolation threshold is apparently well predicted by Flory’s theory of the gel point for dendritic polymerization.