Local order in water: The fifth neighbor

We exploited the possibility to unfold the oxygen-oxygen pair-correlation function of liquid water modeled by the BK3 force field into distributions of the first, second, third, etc. neighbors of the central water molecule. Especially, we were interested in the 5th neighbor because its position is halfway between the first and the second coordination spheres. We found that each molecule has three tightly hydrogen bonded neighbors, a loosely bonded 4th neighbor and sometimes a non-tetrahedral 5th molecule which is a proton donor. We determined lifetimes of first shell molecules, the average reorientation or “jump” time, and the time needed to change the order of neighbors. Certain molecules can be 5th neighbors of more centers which may create multiple overlaps of first coordination shells. The density increase of water relative to hexagonal ice is caused by these overlaps and the hydrogen bond of the 5th molecule. This possibility also plays a role in fluctuations or makes possible the adaptation to high external pressure as well.