Dr Biswajit Santra, Postdoctoral Research Scholar in the Laboratory of Professor Roberto Caro, Department of Chemistry, Princeton University, Princeton, NJ, USA, presents:
In nature ambient liquid water is denser than its solid counterpart, crystalline ice, an anomalous property of water that has enormous impact on the environment and life. However, semi-local (GGA) density functional theory (DFT) based ab initio molecular dynamics fail to predict this important property of water.
We have found that this discrepancy arises due to the lack of van der Waals (vdW) interactions in GGA-DFT. In fact, an accurate description of vdW interactions is crucial in stabilizing liquid water configurations with an increased population of molecules in the interstitial region (i.e., the region between the first two coordination shells), thereby increasing the density by ~15% with respect to ambient liquid water at the GGA-DFT level of theory.
Furthermore, by simultaneously accounting for both exact exchange and vdW interactions, the computed equilibrium densities of both liquid water and crystalline ice were found to be within a few percent of the experimentally observed densities—a level of accuracy that has been difficult to achieve from DFT simulations of water to date. With such accurate potentials, we further attempt to predict and characterize the anomalous density maximum that is observed in liquid water just above the melting temperature.