The use of ab initio molecular dynamics (AIMD) based on semi-local density functional theory (DFT) to simulate ambient liquid water is challenging and has several shortcomings. In particular, the anomalous density ordering between ambient liquid water and ice is simply not observed from simulations utilizing semi-local (GGA) approximations for the underlying DFT exchange-correlation functional. In this work, we have performed a series of AIMD simulations in the isothermal-isobaric (NpT) ensemble in order to determine the individual and collective effects of exact exchange (via the use of the PBE0 hybrid functional [1]) and van der Waals (vdW) interactions (via the use of a pairwise additive C6/R6correction [2]) on the equilibrium densities of ambient liquid water and ice. We have found that the inclusion of vdW interactions is crucial in obtaining a liquid water which is denser than ice, its solid counterpart. The prediction of this density ordering is achieved primarily due to the fact that vdW interactions stabilize liquid water configurations with an increased population of water 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. Relatively speaking, the influence of exact exchange on the density of ambient liquid water is rather small; however, the treatment of exact exchange, which reduces the self-interaction error in GGA-DFT, is important for obtaining oxygen-oxygen (O-O) pair correlation functions that are in much closer agreement with experiment. Therefore, by simultaneously accounting for both exact exchange and pairwise vdW interactions, the computed equilibrium densities of both liquid water and ice are 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.[1] X. Wu, A. Selloni, and R. Car, Phys. Rev. B 79 , 085102 (2009). [2] A. Tkatchenko and M. Scheffler, Phys. Rev. Lett. 102 , 073005 (2009).*This work received funding from the Department of Energy under Grant Nos.: DOE DE-SC0008626 and DOE DE-FG02ER46201.**This research used resources of the Argonne Leadership Computing Facility at Argonne National Laboratory, which is supported by the Office of Science of the U.S. Department of Energy under contract DE-AC02-06CH11357.

Predicting the anomalous density ordering between liquid water and ice using <i>ab initio</i> molecular dynamics

248th National Meeting (2014)

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Predicting the anomalous density ordering between liquid water and ice using ab initio molecular dynamics