While the real-space structure of solvation shells has been explored for decades, a dynamical perspective that directly relies on changes in the H-bond network became accessible more recently mainly via far-infrared (THz) spectroscopies. A remaining key question is how many hydration shells are affected by ion-induced network perturbations. We disclose that theoretical THz difference spectra of aqueous salt solutions can be deciphered in terms of only a handful of dipolar auto- and cross-correlations, including the second solvation shell. This emphasizes the importance of cross-correlations being often neglected in multicomponent models. Analogously, experimental THz responses of simple ions can be deciphered in a similar way. Dramatic intensity cancellations due to large positive and negative contributions are found to effectively shift intensity maxima. Thus, THz spectroscopy provides an unprecedented view on the details of hydration dynamics, which can be understood by a combination of experiment and theory.

On the basis of ab initio Gibbs ensemble Monte Carlo simulations, we map the liquid-vapor phase diagram of water described by the RPBE density functional supplemented by D3 dispersion corrections and estimate the critical point by density extrapolation. Knowing the approximate location of the critical point, two sets of ab initio molecular dynamics simulations at gas-like and liquid-like densities deep in the supercritical phase of water are carried out where particular attention is payed to ergodic sampling in view of large correlation lengths and long correlation times. Structural, H-bonding, and dipolar properties of RPBE-D3 water are analyzed along the liquid-vapor coexistence curve upon moving toward the critical point and compared to those in the supercritical state. The properties of high-density supercritical water are astonishingly similar to those of the liquid on the coexistence curve under subcritical conditions at comparable density. Upon decomposing the molecular dipole moments into purely configurational and electronic polarization/charge-transfer contributions, it is demonstrated that the latter play a decreasing role in liquid water upon approaching the critical point on the coexistence curve. Moreover, these many-body effects are systematically suppressed in supercritical water due to the significantly reduced H-bonding network.

Evolution of water's structural dynamics from ambient liquid to supercritical dense liquid-like and dilute gas-like conditions.

Thinking about water is inextricably linked to hydrogen bonds,w hicha re highly directional in character and determine the unique structure of water,i np articular its tetrahedral H-bond network. Here,w ea ssess if this common connotation also holds for supercritical water.W ee mploy extensive ab initio molecular dynamics simulations to systematically monitor the evolution of the H-bond network mode of water from room temperature,w here it is the hallmark of its fluctuating three-dimensional network structure,t os upercritical conditions.O ur simulations reveal that the oscillation period required for H-bond vibrations to occur exceeds the lifetime of H-bonds in supercritical water by far.I nstead, the corresponding low-frequency intermolecular vibrations of water pairs as seen in supercritical water are found to be well represented by isotropic van-der-Waals interactions only. Based on these findings,w ec onclude that water in its supercritical phase is not aH-bonded fluid.

The supercritical phase of fluids has long been known to feature significantly different liquidlike and gaslike regimes. However, it is textbook knowledge that the supercritical state is a homogeneous fluid phase where properties change continuously. Nevertheless, there has been an increasing amount of evidence published that suggests that there might exist a unique line that rigorously separates different regimes in supercritical phases, particularly in the case of water. Here, we use the quasiexact IAPWS95 equation of state to rigorously assess the macroscopic thermodynamic properties of supercritical water without invoking any water model or related approximations. We focus on how these properties change deep in the supercritical phase, in particular if they allow one to introduce a unique "thermodynamic separatrix." Our rigorous thermodynamic analysis, which relies exclusively on accurate experimental data, makes clear that there is no unique separatrix in real supercritical water-such as the recently much-invoked "Widom line." A comparison to the van der Waals equation of state reproduces qualitatively all our findings for real water, thereby suggesting that our analysis should be transferable to other fluids and critical points. Topological analysis of the H-bond network structure of supercritical water, as obtained from molecular-dynamics simulations using a standard water model, demonstrates that also the percolation line does not provide a meaningful separatrix to rigorously distinguish liquidlike from gaslike regimes.