Density Fluctuations in Liquid Water

The density distributions and fluctuations in grids of varying size in liquid water at ambient pressure, both above the freezing point and in the supercooled state, are analyzed from the trajectories obtained from large-scale molecular dynamics simulations. It is found that the occurrence of low- and high-density regions (LDL and HDL) is transient and their respective residence times are dependent on the size of the simulated system. The spatial extent of density-density correlation is found to be within 7 Å or less. The temporal existence of LDL and HDL arises as a result of natural density fluctuations of an equilibrium system. The density of bulk water at ambient conditions is homogenous.

Figure 1

Normalized probability distributions of density in differently sized grids (with number of $n$ grids per overall 316 Å cubic box length indicated in parenthesis) at 270 K and 1 bar pressure. Results are very similar at 200 and 225 K and for the small system (see text). The data for the 9.9 Å-side elements in the small system are shown, but omitted for clarity in other cases.

Figure 2

Normalized grid-based spatial density-density correlation function at 270 K in terms of distance between the centers of the elements in ${32}^3$ and ${64}^3$ grids for the $1.07\times {10}^6$-molecule system. The lack of correlation is evident between elements whose centers are more than around 8 Å distant. This varies little with temperature.

Figure 3

Normalized probability distributions of persistence times of high-densities in 9.9 Å-side grid elements in the $1.07\times {10}^6$-molecule and small systems; these are defined as periods of time of density greater than $1.1\mathrm{g}/{\mathrm{cm}}^3$. The distributions for low-density residence times are essentially identical.

Figure 4

Normalized probability distributions of overall proportion of simulation time for which low- and high-densities are present in 9.9 Å-side grid elements in the $1.07\times {10}^6$-molecule system.