Ab Initio Theoretical Study of Water: Extension to Extreme Conditions

Abstract

The electronic, liquid structure of water and its thermodynamic properties including the ionic product (pK _w) are studied over a wide range of temperatures and densities based on ab initio molecular orbital theory combined with the integral equation method for a molecular liquid. For the neat liquid system, it is found that the molecular dipole moments and electronic polarization energies of a water molecule decrease with increasing temperature and/or density, being in quantitative accord with the experimental data determined based on the NMR chemical shift coupled with molecular dynamics simulation. The temperature and density dependence of the number of hydrogen-bonds is discussed in terms of the liquid structure of water. The pK _w obtained from the theory shows a monotonical decrease with increasing density at all the temperatures investigated, in good accord with experimental observation.