title: How van der Waals interactions determine the unique properties of water.
abstract: Whereas the interactions between water molecules are dominated by strongly directional hydrogen bonds (HBs), it was recently proposed that relatively weak, isotropic van der Waals (vdW) forces are essential for understanding the properties of liquid water and ice. This insight was derived from ab initio computer simulations, which provide an unbiased description of water at the atomic level and yield information on the underlying molecular forces. However, the high computational cost of such simulations prevents the systematic investigation of the influence of vdW forces on the thermodynamic anomalies of water. Here, we develop efficient ab initio-quality neural network potentials and use them to demonstrate that vdW interactions are crucial for the formation of water's density maximum and its negative volume of melting. Both phenomena can be explained by the flexibility of the HB network, which is the result of a delicate balance of weak vdW forces, causing, e.g., a pronounced expansion of the second solvation shell upon cooling that induces the density maximum.
authors.affiliation: Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, D-44780 Bochum, Germany; tobias.morawietz@theochem.ruhr-uni-bochum.de joerg.behler@theochem.ruhr-uni-bochum.de.
authors.forename: Tobias
authors.initials: T
authors.lastname: Morawietz
authors.affiliation: Faculty of Physics, University of Vienna, A-1090 Vienna, Austria.
authors.forename: Andreas
authors.initials: A
authors.lastname: Singraber
authors.affiliation: Faculty of Physics, University of Vienna, A-1090 Vienna, Austria.
authors.forename: Christoph
authors.initials: C
authors.lastname: Dellago
authors.affiliation: Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, D-44780 Bochum, Germany; tobias.morawietz@theochem.ruhr-uni-bochum.de joerg.behler@theochem.ruhr-uni-bochum.de.
authors.forename: Jörg
authors.initials: J
authors.lastname: Behler
keywords: ab initio liquid water; density-functional theory; neural network potentials; van der Waals interactions; water structure