Structure in liquid water: A study of spatial distribution functions

Abstract

Despite the fact that an enormous literature has now accumulated on the structure in liquid water, the focus has been primarily limited to the average radial distributions of particles; local (atomic) pair-density maps which span both the radial and the angular coordinates of the separation vector have remained largely unexplored. In this work, we have obtained the spatial distribution functions gOO(r,Ω) and gOH(r,Ω) for liquid water and have applied them to an analysis of the equilibrium structure. Molecular dynamics simulations of SPC/E water have been carried out at temperatures of −10, 25, and 100 °C and the local liquid structure examined. It is found that the unfolded O...O distribution demonstrates, in addition to peaks consistent with a continuous tetrahedral network pattern, a distinct maximum in the local atomic pair density at ‘‘interstitial’’ separations of about 3.5 Å. This local maximum is lost in the spatially folded radial distribution function gOO(r) due to averaging over the entire angular space. By examining the peaks in gOO(r,Ω) due to nearest neighbors, we have shown that the tetrahedral network coordination number in liquid SPC/E water equals 4.0 and does not depend on temperature. The average number of molecules in additional nontetrahedral coordination, which is found to vary with temperature, has also been extracted, enabling us to establish full average coordination numbers of 4.8–5.0 in the temperature range of −10–100 °C. In addition, we have determined and analyzed statistical distributions for the pair energies and H-bond angles for different water fractions as identified from gOO(r,Ω) and gOH(r,Ω).

Polar stacking of molecules in liquid chloroform

“…These highlight volumes where the pair-correlation function takes large values and contains specified fractions of the atoms. 12 The dashed circle in Figure 2b indicates the upper C-H distance limit used for the construction of the C-H SDF shown in Figure 2c. Again, it can be seen that the most likely position of H atoms is in -z 1 direction below the fully chlorinated face of the reference molecule.…”

Structure of the nanobubble clusters of dissolved air in liquid media

“…Such a model was apparently first put forward in literature in the known study [52] and then was developed in the works [53][54][55][56][57][58][59][60][61]. Within the framework of this approach it is possible to explain many anomalies of liquid water (it is possible to find details on the website [62]).…”

“…c , where we used (57), (58). After substituting u 2 2 1/2 instead of u into formula (53), we arrive at…”

Using DL_POLY to study the sensitivity of liquid structure to potential parameters

“…This is confirmed by a steady decrease in the tetrahedral order parameter in this family (see Table 2). Figure 2 shows the three dimensional contours of probability functions for finding oxygen sites around a water molecule (20). In the three members of the hybrid family (left hand column -SPC/E, s15 and s30) the most probable positions are along the OH bonds.…”

“…In SPC/E water there are two other regions of high water density behind the oxygen, giving an approximate tetrahedral arrangement of neighbouring water molecules in the first solvation shell. At lower probabilities (not shown) one sees a build-up of probability above and below the OH bonds which Svishchev and Kusalik (20) interpreted as corresponding to a fifth water molecule penetrating the inner shell. In the s15 model the pattern is similar, although the localisation of the neighbouring molecules is less, with rather diffuse regions behind the molecule.…”