Structure and phase diagram of high-density water: the role of interstitial molecules.

The structural transformations occurring to water from the low- to the high-density regimes have been studied by classical molecular dynamics calculations. The local structure is analyzed through a proper choice of the relevant orientational distribution functions. This approach sheds light on the key role played by the interstitial molecules in the second coordination shell and identifies a clear structural fingerprint of high-density water. As a consequence, the analogy between the structure of high-density water and those of high-density ices is evidenced.