Thermodynamic and structural properties of liquid water around the temperature of maximum density in a wide range of pressures: A computer simulation study with a polarizable potential model

Computer simulations of liquid water have been performed with the polarizable Brodholt-Sampoli-Vallauri (BSV) potential model at several temperatures around the temperature of maximum density (TMD) in the entire pressure range in which such a density maximum exists in thermodynamically stable liquid water. The temperature and pressure dependence of the thermodynamic and structural properties has been analyzed on the basis of these simulations. We find that the BSV model reproduces most of the important thermodynamic features of water in this temperature and pressure range. The BSV model is also found to reproduce another of the anomalous properties of liquid water, i.e., the isothermal compressibility goes through a minimum when the temperature is increased. On the other hand, it is found that above the TMD the density of the model decreases much faster with increasing temperature than in real water. However, this failure, which is a common feature of the polarizable water models, is rather unimportant in the narrow temperature range studied here. In analyzing the molecular level structure of water as a function of the thermodynamic conditions we find that the increase of the temperature as well as of the pressure has a distorting effect on the tetrahedral hydrogen bonded network, and it causes an increase of the fraction of the interstitial neighbors of the molecules. These changes result in a more compact structure and hence in an increase of the density of the system. When these changes are induced by the temperature, the increasing thermal motion of the molecules can compensate the increase of the density, and the two opposite effects result in the appearance of the density maximum. (C) 2001 American Institute of Physics.