Ab initio study of water. II. Liquid structure, electronic and thermodynamic properties over a wide range of temperature and density

Abstract

The electronic and liquid structures of water and its thermodynamic properties are studied over a wide range of temperature (0°–600 °C) and density (0.6–1.4 g/cm3) based on the ab initio molecular orbital theory combined with the integral equation method of liquid. Unlike standard treatments of water by means of the classical statistical mechanics including molecular simulations, the effective charges on oxygen and hydrogen atoms in water molecules are not “input parameters,” but naturally predicted from the theory in the course of self-consistent determination of the electronic structure and the density pair correlation functions in liquids. It is found that the molecular dipole moments and electronic polarization energies decrease with increasing temperature and/or density. The theoretical results for dipole moments are in quantitative accord with the experimental data, which has been determined based on the NMR chemical shift coupled with the molecular dynamics simulation [N. Matsubayashi, C. Wakai, and M. Nakahara, J. Chem. Phys. 110, 8000 (1999)]. The state dependence of the electronic structure is discussed in terms of the thermal activation of molecules and intermolecular interactions including the hydrogen bonds. The liquid structure of water is discussed in the wide range of thermodynamic states in terms of S(r), an average response of the pair correlation functions to temperature change which is introduced in the present study in order to make structural characteristics of water more distinctive. It is concluded from the behavior of the function that the short range structure of water retains the characteristics to ice, or the tetrahedral configuration, over relatively wide range of temperature in the normal density (1.0 g/cm3). The ice-like characteristics of water disappear to large extent both at high (1.4 g/cm3) and low (0.6 g/cm3) densities for different reasons: in the high density due to the packing effect, while in the low density due to essentially entropic cause, or increased configuration space available to a molecule. The distance between the nearest-neighbor molecules in water are insensitive to the density change compared with those corresponding to the Lennard-Jones fluid. The difference in the behaviors between the two fluids is explained in terms of the intermolecular interactions and liquid structures. The number of hydrogen bonds is calculated from the oxygen–hydrogen pair correlation function using a new definition based on S(r), which enables us to distinguish the hydrogen-bonded OH pairs from those just in contact due to packing effect. The temperature and density dependence of the quantity is discussed in terms of the liquid structure of water.