CAN THE DENSITY MAXIMUM OF WATER BE FOUND BY COMPUTER-SIMULATION

Path integral molecular dynamics (PIMD) simulations of the extended si mple point charge (SPC/E) model of liquid water (both H2O and D2O) hav e been performed in order to find the temperature at which the maximum liquid density is obtained. For comparison, purely classical SPC and SPC/E water have also been simulated over long periods (more than 200 ps). Structural properties and various temperature dependent quantitie s are reported. Special attention has been given to the fluctuations o f pressure and volume in simulations run in the canonical (constant NV T) and isothermal-isobaric (constant NPT) ensembles, respectively. Alt hough a density maximum is detected by monitoring energy-volume correl ations as a function of temperature, the large statistical uncertainty in the correlations reduces the significance of this finding. The mai n conclusions of this work are threefold. First, the use of the energy -volume correlation moment to determine the density maximum appears to be a useful method, although a number of very long simulations of bet ween 0.5-1.0 ns over a fairly wide temperature range are necessary to obtain accurate results. Second, neither the SPC nor the SPC/E water m odels ate able to reproduce the density of cold water accurately. Fina lly, a quantum mechanical treatment of the water model is necessary to correctly represent the structure, energy, and fluctuations of the co ndensed phase.