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1. Liquid–liquid criticality in the WAIL water model

   https://doi.org/10.1063/5.0099520  

   Weis, Jack; Sciortino, Francesco; Panagiotopoulos, Athanassios Z.; Debenedetti, Pablo G. (July 2022, The Journal of Chemical Physics)

   The hypothesis that the anomalous behavior of liquid water is related to the existence of a second critical point in deeply supercooled states has long been the subject of intense debate. Recent, sophisticated experiments designed to observe the transformation between the two subcritical liquids on nano- and microsecond time scales, along with demanding numerical simulations based on classical (rigid) models parameterized to reproduce thermodynamic properties of water, have provided support to this hypothesis. A stronger numerical proof requires demonstrating that the critical point, which occurs at temperatures and pressures far from those at which the models were optimized, is robust with respect to model parameterization, specifically with respect to incorporating additional physical effects. Here, we show that a liquid–liquid critical point can be rigorously located also in the WAIL model of water [Pinnick et al., J. Chem. Phys. 137, 014510 (2012)], a model parameterized using ab initio calculations only. The model incorporates two features not present in many previously studied water models: It is both flexible and polarizable, properties which can significantly influence the phase behavior of water. The observation of the critical point in a model in which the water–water interaction is estimated using only quantum ab initio calculations provides strong support to the viewpoint according to which the existence of two distinct liquids is a robust feature in the free energy landscape of supercooled water. 

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2. Correlated libration in liquid water

   https://doi.org/10.1063/5.0200094  

   Shelton, David P. (March 2024, The Journal of Chemical Physics)

   The libration spectrum of liquid H2O is resolved into an octupolar twisting libration band at 485 cm−1 and dipolar rocking–wagging libration bands at 707 and 743 cm−1 using polarization analysis of the hyper-Raman scattering (HRS) spectrum. Dipole interactions and orientation correlation over distances less than 2 nm account for the 36 cm−1 splitting of the longitudinal and transverse polarized bands of the dipolar rocking–wagging libration mode, while the intensity difference observed for the bands is the result of libration correlation over distances larger than 200 nm. The coupled rock and wag libration in water is similar to libration modes in ice. The libration relaxation time determined from the width of the spectrum is 36–54 fs. Polarization analysis of the HRS spectrum also shows long range correlation for molecular orientation and hindered translation, bending and stretching vibrations in water. 

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3. Ice–Liquid Oscillations in Nanoconfined Water

   https://doi.org/10.1021/acsnano.8b03403  

   Kastelowitz, Noah; Molinero, Valeria (July 2018, ACS Nano)
4. Hydrogen bond network modes in liquid water

   https://doi.org/10.1103/PhysRevb.108.174203  

   Shelton, David P. (November 2023, Physical Review B)

   Collective modes and dynamics of dense disordered materials such as water are not well understood, but nonlinear optics provides a sensitive probe to study polar modes in these materials. Here we report the hyper- Raman (HRS) light scattering spectrum measured for liquid D2O decomposed into contributions from transverse and longitudinal dipolar modes and octupolar modes, using the polarization dependence of HRS. Transverse HRS is observed for orientation and stretching modes, while longitudinal HRS is observed for translation, libration, and bending modes. The HRS observations indicate molecular correlation at distances >200 nm for all modes of orientation, libration, and vibration. The rocking/wagging, twisting, and translation modes for D2O molecules in the hydrogen bonded network are distinguished. The LO-TO splitting is 28 cm−1 for the libration mode and 16 cm−1 for the translation mode, and the relaxation time for libration modes is about 80 fs. The long-range correlation of the orientation and stretching modes is explained as the result of the dipole-dipole orientation correlation in a dipolar fluid. The long-range correlation of the longitudinal polarized libration and bending modes needs further study. 

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5. Molecular origins of bulk viscosity in liquid water

   https://doi.org/10.1039/d0cp01560a  

   Yahya, Ahmad; Tan, Luoxi; Perticaroli, Stefania; Mamontov, Eugene; Pajerowski, Daniel; Neuefeind, Joerg; Ehlers, Georg; Nickels, Jonathan D. (May 2020, Physical Chemistry Chemical Physics)

   null (Ed.)

   The rapid equilibrium fluctuations of water molecules are intimately connected to the rheological response; molecular motions resetting the local structure and stresses seen as flow and volume changes. In the case of water or hydrogen bonding liquids generally, the relationship is a non-trivial consideration due to strong directional interactions complicating theoretical models and necessitating clear observation of the timescale and nautre of the associated equilibrium motions. Recent work has illustrated a coincidence of timescales for short range sub-picosecond motions and the implied timescale for the shear viscosity response in liquid water. Here, neutron and light scattering methods are used to experimentally illustrate the timescale of bulk viscosity and provide a description of the associated molecular relaxation. Brillouin scattering has been used to establish the timescale of bulk viscosity; and borrowing the Maxwell approach, the ratio of the bulk viscosity, ζ , to the bulk modulus, K , yields a relaxation time, τ B , which emerges on the order of 1–2 ps in the 280 K to 303 K temperature range. Inelastic neutron scattering is subsequently used to describe the motions of water and heavy water at the molecular scale, providing both coherent and incoherent scattering data. A rotational (alternatively described as localized) motion of water protons on the 1–2 ps timescale is apparent in the incoherent scattering spectra of water, while the coherent spectra from D 2 O on the length scale of the first sharp diffraction peak, describing the microscopic density fluctuations of water, confirms the relaxation of water structure at a comparable timescale of 1–2 ps. The coincidence of these three timescales provides a mechanistic description of the bulk viscous response, with the local structure resetting due to rotational/localized motions on the order of 1–2 ps, approximately three times slower than the relaxations associated with shear viscosity. In this way we show that the shear viscous response is most closely associated with changes in water network connectivity, while the bulk viscous response is associated with local density fluctuations. 

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