Structural and Thermophysical Anomalies of Liquid Water: A Tale of Molecules in the Instantaneous Low- and High-Density Regions.

Water is believed to be a heterogeneous liquid comprising multiple density regions that arise because of the presence of interstitial molecules and can be differentiated by their structure as well as the existence of hydrogen-bonded pairs with varying strengths. First-principles molecular dynamics studies were performed at six different temperatures to investigate the effect of temperature on the thermophysical, structure, dynamics, and vibrational spectral properties of the water molecules using dispersion-corrected density functional theory. The variation of properties like density, cohesive energy, and compressibility with a change in temperature produces a trend that matches with the experiments and resembles the experimentally observed anomalous behavior. We explore the possibility of explaining the trends in calculated properties by analyzing the structure and dynamics of the water molecules in terms of instantaneous low- and instantaneous high-density regions that are found during the simulation time. The dynamics of these two types of water molecules were studied by calculating the lifetime from the proposed autocorrelation functions. The lifetime of formation of instantaneous low-density water is found to decrease with an increase in temperature, whereas the lifetime of instantaneous high-density water is found to be maximum at 298 K among all the considered temperatures. The presence of more interstitial water molecules is observed at this temperature. The signature of these water molecules is found in the radial distribution function, spatial distribution function, void distribution, configurational space, orientational dynamics, and spectral diffusion calculations. It is also found that around 298 K, these water molecules are present distinctively that mix up with the first and second solvation shells with the rise of the temperature. The outlook of the reported results can be extended to other thermodynamic conditions to explain some of the anomalous properties, which can be related to the presence of the interstitial molecules in water.