Molecular Model Of Water Research Articles

Interplay between Molecular Simulation and Neutron Scattering in Developing New Insights into the Structure of Water

For three decades, molecular models for water, nature's most important liquid, have been developed and refined by fitting to structure measured by neutron scattering. The decade-old widely accepted structure of water at room temperature and pressure was recently revised as a byproduct of attempts to understand the structure of high-temperature/high-pressure water for which, in a remarkable reversal of roles, molecular models successfully pinpointed inaccuracies in scattering data. Subsequent improvements in analyzing scattering data have led to reevaluation of water structure at normal conditions. This remarkable interplay between molecular modeling and experiment suggests molecular methods can effectively complement scattering experiments.

Dependence of Hydration Free Energy on Solute Size

The dielectrically consistent reference interaction site model theory (DRISM) was used to calculate excess chemical potentials of solvation for the immersion of a nonpolar solute molecule (Lennard-Jones sphere) in a molecular model of water in a wide variety of state conditions. The chemical potential was found to be predominantly entropic. In all cases the chemical potential was found to consist almost entirely of a solute surface area and a solute volume term. Both these terms were significant over a range of solute sizes and pressure/temperature states. It was concluded that the volume-dependent term must include contributions in addition to that from the system pressure. An exactly solvable lattice model of solvation was also investigated, and the model conditions for which the chemical potential becomes predominantly entropic were determined. One situation was shown where a volume-dependent entropic term in the chemical potential, other than pressure, arises.

Liquid densities and structural properties of molecular models of water

We have calculated the density of model water potentials—employing an effective pairwise additive potential (RER) and an ab initio-derived many-body potential including polarizabilities (NEMO)—as a function of temperature at constant pressure. It is found that neither model can quantitatively reproduce the density of liquid water between 250 and 373 K. More disturbingly, neither potential exhibits a density maximum. Modifying the effective potential to give the correct density at different temperatures reveals subtle changes in the importance of different water configurations. These configurational changes are shown as possible causes of the anomalous density behavior of liquid water.

Hydrogen bonding in supercritical water

We study the hydrogen bonding structure of water models at supercritical conditions by molecular dynamics to directly compare with recent microstructural data obtained by neutron diffraction with isotopic substitution (NDIS) experiments. We also study the angular dependence of the hydrogen–oxygen pair distribution function to gain insight into the hydrogen bonding mechanism in the molecular models for water. The simulation results suggest that the angle-averaged radial distribution function gOH(r) measured by NDIS experiments may not provide a complete picture of the degree of hydrogen bonding.

Molecular models of water and neutral solutes against a charged wall

Molecular models of water and neutral solutes against a charged wall