The three site-site partial structure factors for water have been measured as a function of pressure, using neutron diffraction, at a temperature of 268 K. It is found that the measured structure functions imply a continuous transformation with increasing pressure from a low-density form of water ( ${\rho }_L\sim 0.0295\mathrm{molecules}/{Å}^3$), with an open, hydrogen-bonded tetrahedral structure, to a high-density form of water ( ${\rho }_H\sim 0.0402\mathrm{molecules}/{Å}^3$), with nontetrahedral O-O-O angles and a collapsed second coordination shell, which implies broken hydrogen bonds between the first and second coordination shells.

Recent controversy in the literature about the structure of water away from ambient conditions is manifested by significant differences between the site–site pair correlation functions of water as derived from neutron diffraction data and the same quantities obtained in computer simulations using an effective pairwise potential for the intermolecular interactions. One possible explanation of the discrepancies between computer models of water and neutron diffraction results near the critical point is that they arise from uncertainties in the inelasticity correction, which is particularly large for light water. To test out this idea, a new method of obtaining the pair correlations is described. This new analysis method is applied both to the earlier neutron data on non-ambient water and on recently reported data for a range of densities at 573 K. For this newer data the incident neutron spectrum is derived from an ambient water moderator instead of the liquid methane $\text{(100}$ K) moderator of previous work, and this has the effect of altering the shape of the inelasticity distortion to the diffraction data. It is found that the resulting site–site correlation functions from different experiments are now closely consistent with one another, indicating that the inelasticity correction has been successfully performed by this new analysis. It is also demonstrated that the new site–site correlation functions can be generated from three-dimensional distributions of water molecules. The previously reported discrepancies between simulation and neutron data are not as severe with this new analysis, but there remain systematic trends with density and temperature which apparently are not reproduced by computer simulation with empirical pair potentials.

The ion induced modification to the tetrahedral structure of water is a topic of much current interest. We address this question by interpreting neutron diffraction data from monovalent ionic solutions of NaCl and KCl using a computer assisted structural modeling technique. We investigate the effect that these ions have on the water–water O–O, O–H and H–H radial distribution functions as a function of ionic concentration. It is found that the O–H and H–H functions are only marginally affected by ionic composition, signaling that hydrogen bonding between water molecules remains largely intact, even at the highest concentrations. On the other hand the O–O functions are strongly modified by the ions. In particular the position of the second peak in g_OO(r), is found to move inwards with increasing salt concentration, in a manner closely analogous to what happens in pure water under pressure. Furthermore by recalculating g_OO(r) after excluding all the water molecules in the first hydration shell of each ion, we show that this structural perturbation exists outside the first hydration shell of the ions.

The structure of water has been analyzed at eight different thermodynamic states from ambient to supercritical conditions both by molecular dynamics (MD) and Reverse Monte Carlo (RMC) simulation. MD simulations have been carried out with two different potential models, a polarizable potential and one of the most successful nonpolarizable models, i.e., the well known Simple Point Charge potential in its revised version labeled by $E$ (SPC/E). It has been found that, although the polarizable model can reproduce the experimental partial pair correlation functions at the high temperature states better than the nonpolarizable one, it still cannot account for all the features of the measured functions. The experimental partial pair correlation functions have been well reproduced by the RMC simulations at every state point. The resulting structures have been analyzed in detail. It has been found that the tetrahedral orientation of the hydrogen bonded neighbors is already lost at 423k, whereas the hydrogen bonds themselves remain preferentially linear even above the critical point. In investigating the properties of the hydrogen-bonded clusters of the molecules it has been found that the space-filling percolating network, present under ambient conditions, collapses around the critical point.