Water has been the subject of numerous experimental and theoretical investigations, given its paramount importance in the physical sciences and its key role in life. Nevertheless, many fundamental questions about the properties of water are yet unanswered. For example, most investigations of the effect of compression on the microscopic structure of liquid water have been performed at low pressures (< 0.1 GPa), and only a small number of experiments have been performed at pressures up to 2 GPa. For pressures larger than 2 GPa, information on the liquid is even more limited; site-site pair correlation functions and structure factors have not been measured, and structural data are only available from simulations based on empirical potentials. We have investigated the bulk properties of liquid water by first principles molecular dynamics simulations of a cubic box containing 54 molecules, subjected to periodic boundary conditions. The nuclear motion was described by Newtonian dynamics, and many body interactions between electrons were described by density functional theory in the local density approximation, and with the PBE generalized gradient approximation.

Figure 1. Radial distribution functions of water at ambient conditions. The black lines correspond to the simulation data, and the dashed lines are from a recent neutron diffraction experiment (Soper, 1996).

In Fig. 1, the radial distribution functions (RDFs) at ambient conditions are displayed, along with recent neutron diffraction results. Overall, the agreement is very good. The small differences between the measured and computed RDFs are most likely due to the neglect of proton quantum effects in our simulations.

Figure 2. The effect of a) temperature, and b) pressure on the RDFs of liquid water.

Having established the reliability of our model at ambient conditions, we then carried out a series of simulations on liquid water at elevated temperatures and pressures. In Fig. 2a, the effect of an increase in temperature from 300K to 600K on the RDFs is shown. Although the intensities of the peaks are reduced, the peak positions remain essentially fixed, with a small outward shift of the first peak in the oxygen-oxygen RDF. An increase in pressure to 10 GPa is shown in Fig. 2b to have a much larger effect on the liquid structure. At this high pressure, each water molecule is closed packed and surrounded by 12.9 molecules, as opposed to 4.5 neighbors at ambient conditions. The changes in the atomic structure, which cause a disruption of the hydrogen bond network, are accompanied by sizeable changes in the electronic density as well. These simulations provide new predictions on the compressed liquid structure, which will be useful for comparisons with future experiments.

Figure 3. Isosurfaces of the electron density in the liquid water simulations. The panel to the left corresponds to the simulation at ambient conditions and the panel to the right is at high pressure and temperature. The arrows indicate significant differences in the hydrogen bonding as pressure is applied.

UCRL-MI-140705
Date last modified: 10/09/00
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