Fluctuation of local order and connectivity of water molecules in two phases of supercooled water

We investigate the fluctuations of coordination numbers and the spatial correlations of imperfectly coordinated water molecules in two phases of supercooled water. It is found, although the average coordination numbers are 4 in local energy minimum structures of both phases, that the magnitude of the fluctuation differs significantly between two phases. Connectivity of water molecules whose first and second neighbors are also perfectly (four-) coordinated is examined. It is revealed that such locally ordered water molecules spread over the entire system in the low density liquid phase whereas those molecules form only small size clusters in the high density liquid water phase.