On the microscopic structure of liquid water

The radial distribution function of liquid water, as obtained by the computer simulations of several classical models of water, is re-examined herein and shown to display two intriguing features. These consist of a compact 'three-peaks structure' over three molecular diameters, which is followed by an apparent loss of the packing correlations beyond R-c approximate to 9 angstrom. This is in contrast to simple liquids for which the correlations decay continuously with distance. This structure is reproduced for many widely used classical force field models of water and by scattering experiments as well. It is also preserved in aqueous binary mixtures of organic solvents, even up to equimolar mixture in some cases. The analysis of the structure factor highlights the role played by the competition between the packing effect and the hydrogen bonding interactions. This analysis, in terms of competition of two length scales, is also supported by a simple core-soft model, which reproduces the structural features outlined above.