A molecular level explanation of the density maximum of liquid water from computer simulations with a polarizable potential model

Differences in the structure of water are investigated on the basis of a re cent set of Monte Carlo simulations with a polarizable water model at tempe ratures corresponding to the same density below and above the density maxim um. The simulations reproduced well the experimental differential pair corr elation function of the molecular centers and its running coordination numb er. It is shown that with increasing temperature an increasing number of mo lecules leaves the tetrahedral hydrogen-bonded network. These interstitial molecules are located in the cavities of the tetrahedral network of the oth er molecules, forming closely packed structural units with their neighbours . The effect of the increasing number of these closely packed patches on th e density of the system can compensate the increasing thermal motion of the molecules up to a certain point. These two opposite effects are shown to b e responsible for the appearance of the density maximum of liquid water at 277 K. (C) 2000 Published by Elsevier Science B.V. All rights reserved.