Fluctuations in ambient water

Abstract

We emphasize the importance in simulations of water of reproducing the isothermal compressibility, dependent on density fluctuations in the liquid, for a correct description of structural fluctuations in simulated water. From x-ray spectroscopy a strict bimodality in terms of fluctuations between high- (HDL) and low-density (LDL) forms of the liquid has been reported. The inherent structure in simulations, i.e. the structure quenched to 0K, is found to be bimodal in terms of HDL and LDL in close correspondence to x-ray spectroscopy. This is, however, smeared out in the real structure where temperature is included. We suggest that the local minima for the HDL and LDL local structures in the simulation need to be deeper and propose that many-body electronic structure effects and non-local van der Waals interactions will be important in this respect. Much larger simulations than hitherto considered are likely necessary to catch the effects of both thermal and structural fluctuations in the simulated liquid. Since structural fluctuations can be expected to significantly affect the dynamics in the liquid it is important that these are taken properly into account when discussing hydrogen-bond breaking and reformation. Here it is essential that the model used correctly reproduces the temperature and pressure dependence in thermodynamic response functions.