Water: cavity size distribution and hydrogen bonds

There are two sizes for water molecules: (a) the distance of closest approach between two hydrogen bonded molecules, 2.8 إ; (b) the distance of closest approach between two non-hydrogen bonded molecules, 3.2 إ, corresponding to the van der Waals diameter of the oxygen atom. This fact is due to the bunching up effect of hydrogen bonds. Correspondingly, when the hydrogen bonding potential is turned off in computer models of water, the effective size of water molecules increases. It is shown by means of scaled particle theory calculations that this basic point has profound effects on the cavity size distribution if the number density is kept constant. The recognition of the bunching up effect of hydrogen bonds is a key factor in order to address the role played by hydrogen bonds in the partitioning of void volume in liquid water.