The structure of liquid water up to 360 MPa from x-ray diffraction measurements using a high Q-range and from molecular simulation

LB Skinner and M Galib and JL Fulton and CJ Mundy and JB Parise and VT Pham and GK Schenter and CJ Benmore, JOURNAL OF CHEMICAL PHYSICS, 144, 134504 (2016).

DOI: 10.1063/1.4944935

X-ray diffraction measurements of liquid water are reported at pressures up to 360 MPa corresponding to a density of 0.0373 molecules per angstrom(3). The measurements were conducted at a spatial resolution corresponding to Qmax = 16 angstrom(-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 angstrom(3) and Q(max) = 20 angstrom(-1) J. Chem. Phys. 138, 074506 (2013) and at 70 degrees C having a density of 0.0327 molecules per angstrom(3) and Q(max) = 20 angstrom(-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 angstrom 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 angstrom, 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. (C) 2016 AIP Publishing LLC.

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