title: Ab initio van der waals interactions in simulations of water alter structure from mainly tetrahedral to high-density-like.
abstract: The structure of liquid water at ambient conditions is studied in ab initio molecular dynamics simulations in the NVE ensemble using van der Waals (vdW) density-functional theory, i.e., using the new exchange-correlation functionals optPBE-vdW and vdW-DF2, where the latter has softer nonlocal correlation terms. Inclusion of the more isotropic vdW interactions counteracts highly directional hydrogen bonds, which are enhanced by standard functionals. This brings about a softening of the microscopic structure of water, as seen from the broadening of angular distribution functions and, in particular, from the much lower and broader first peak in the oxygen-oxygen pair-correlation function (PCF) and loss of structure in the outer hydration shells. Inclusion of vdW interactions is shown to shift the balance of resulting structures from open tetrahedral to more close-packed. The resulting O-O PCF shows some resemblance with experiment for high-density water (Soper, A. K. and Ricci, M. A. Phys. Rev. Lett. 2000, 84, 2881), but not directly with experiment for ambient water. Considering the accuracy of the new functionals for interaction energies, we investigate whether the simulation protocol could cause the deviation. An O-O PCF consisting of a linear combination of 70% from vdW-DF2 and 30% from low-density liquid water, as extrapolated from experiments, reproduces near-quantitatively the experimental O-O PCF for ambient water. This suggests the possibility that the new functionals may be reliable and that instead larger-scale simulations in the NPT ensemble, where the density is allowed to fluctuate in accordance with proposals for supercooled water, could resolve the apparent discrepancy with the measured PCF.
authors.affiliation: Center for Atomic-Scale Materials Design (CAMD), Department, of Physics, Building 307, Nano DTU, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark.
authors.forename: Andreas
authors.initials: A
authors.lastname: Møgelhøj
authors.forename: André K
authors.initials: AK
authors.lastname: Kelkkanen
authors.forename: K Thor
authors.initials: KT
authors.lastname: Wikfeldt
authors.forename: Jakob
authors.initials: J
authors.lastname: Schiøtz
authors.forename: Jens Jørgen
authors.initials: JJ
authors.lastname: Mortensen
authors.forename: Lars G M
authors.initials: LG
authors.lastname: Pettersson
authors.forename: Bengt I
authors.initials: BI
authors.lastname: Lundqvist
authors.forename: Karsten W
authors.initials: KW
authors.lastname: Jacobsen
authors.forename: Anders
authors.initials: A
authors.lastname: Nilsson
authors.forename: Jens K
authors.initials: JK
authors.lastname: Nørskov
mesh_terms: D019281:Dimerization; D006860:Hydrogen Bonding; D056004:Molecular Dynamics Simulation; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water