title: The structure of liquid water up to 360 MPa from x-ray diffraction measurements using a high Q-range and from molecular simulation.
abstract: X-ray diffraction measurements of liquid water are reported at pressures up to 360 MPa corresponding to a density of 0.0373 molecules per Å(3). The measurements were conducted at a spatial resolution corresponding to Q(max) = 16 Å(-1). The method of data analysis and measurement in this study follows the earlier benchmark results reported for water under ambient conditions having a density of 0.0333 molecules per Å(3) and Q(max) = 20 Å(-1) [J. Chem. Phys. 138, 074506 (2013)] and at 70 °C having a density of 0.0327 molecules per Å(3) and Q(max) = 20 Å(-1) [J. Chem. Phys. 141, 214507 (2014)]. The structure of water is very different at these three different T and P state points and thus they provide the basis for evaluating the fidelity of molecular simulation. Measurements show that at 360 MPa, the 4 waters residing in the region between 2.3 and 3 Å are nearly unchanged: the peak position, shape, and coordination number are nearly identical to their values under ambient conditions. However, in the region above 3 Å, large structural changes occur with the collapse of the well-defined 2nd shell and shifting of higher shells to shorter distances. The measured structure is compared to simulated structure using intermolecular potentials described by both first-principles methods (revPBE-D3) and classical potentials (TIP4P/2005, MB-pol, and mW). The DFT-based, revPBE-D3, method and the many-body empirical potential model, MB-pol, provide the best overall representation of the ambient, high-temperature, and high-pressure data. The revPBE-D3, MB-pol, and the TIP4P/2005 models capture the densification mechanism, whereby the non-bonded 5th nearest neighbor molecule, which partially encroaches the 1st shell at ambient pressure, is pushed further into the local tetrahedral arrangement at higher pressures by the more distant molecules filling the void space in the network between the 1st and 2nd shells.
authors.affiliation: X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: L B
authors.initials: LB
authors.lastname: Skinner
authors.affiliation: Physical and Computational Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99354, USA.
authors.forename: M
authors.initials: M
authors.lastname: Galib
authors.affiliation: Physical and Computational Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99354, USA.
authors.forename: J L
authors.initials: JL
authors.lastname: Fulton
authors.affiliation: Physical and Computational Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99354, USA.
authors.forename: C J
authors.initials: CJ
authors.lastname: Mundy
authors.affiliation: Mineral Physics Institute, Stony Brook University, Stony Brook, New York, New York 11794-2100, USA.
authors.forename: J B
authors.initials: JB
authors.lastname: Parise
authors.affiliation: Synchrotron SOLEIL, L'Orme des Merisiers, Saint-Aubin, BP48, 91192 Gif-sur-Yvette, France.
authors.forename: V-T
authors.initials: VT
authors.lastname: Pham
authors.affiliation: Physical and Computational Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99354, USA.
authors.forename: G K
authors.initials: GK
authors.lastname: Schenter
authors.affiliation: X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: C J
authors.initials: CJ
authors.lastname: Benmore