Liquid water is ubiquitous and crucial for environment, biochemistry, electrochemistry, to name a few. Many properties of liquid water are unique compared to other liquids. For example, liquid water is denser than its solid counterpart, ice. When most liquids expand upon heating, liquid water contracts when the temperature rises from 0 ℃ to 4 ℃. Understanding the nature of intermolecular interactions which give rise to such anomalous behavior of water is challenging. We performed density-functional theory (DFT) based ab initio molecular dynamics (AIMD) to investigate the collective effects of electronic interactions on the water anomalies. We find that van der Waals (vdW) interactions—arising from nonlocal electromagnetic fluctuations of non-overlapping electrons—are crucial in densification of liquid water. The importance of vdW is prevalent at long and intermediate inter-atomic distances, and vdW is critical in making liquid water denser than ice. In addition, the short-range covalent bonds in water are also important to accurately describe hydrogen bonds (HBs)—a primary interaction to hold a collection of water molecules together. We find that an accurate balance between HBs and vdW is required to obtain correct solvation shell structures in liquid water.

References:

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