Molecular Simulations of Liquid and Supercritical Water: Thermodynamics, Structure, and Hydrogen Bonding

Water Structure from Scattering Experiments and Simulation

“…The agreement between experiments, and between experiment and simulation, is less good at the supercritical state examined in Figure 7b, although n HB is estimated to be 2.1 by all approaches. The corrected neutron data still show a much stronger trend for loss of hydrogenbonding as a function of temperature and density than does simulation 173,177 . A comparison of more recent neutron data 46 and ab initio molecular dynamics simulations 178 (which agree well with each other), do see greater persistence in hydrogen-bonding at supercritical conditions as judged by a small first peak in g OH (r).…”

“…But because supercritical water has very direct industrial importance, and ample available experimental data against which to compare, the primary emphasis for simulation studies at present is to predict accurate thermodynamic and structural data for liquid phase equilibrium. In fact, well-established empirical two-body force fields do a very reasonable job of reproducing thermodynamic data of phase equilibria far removed from the ambient state 173 . The current activity in simulation of supercritical water is centered around the next generation of polarizable force fields and much greater emphasis on more accurate and predictive water models.…”

“…Overall the twobody potentials are fairly accurate for thermodynamic properties such as densities, isobaric heat capacities, and isothermal compressibility over the range of supercritical temperatures between 573K and 1273K 173 . However, they appear to be less accurate when it comes to reproducing the available partial correlation functions for water structure from neutron scattering over a range of supercritical conditions 179 .…”

“…As water is heated into sub-and super-critical regimes the number of hydrogen-bonds per water molecule has been shown to decrease. At what temperature would the total breakdown of hydrogen-bonding be observed has been debated for many years, with most experimental studies, including Raman 171 , x-ray scattering 47 , and infrared 172 , suggesting that tetrahedral bonding persists to at least ~650K and possibly up to ~770K at 100Mpa 173 .…”

“…However, they appear to be less accurate when it comes to reproducing the available partial correlation functions for water structure from neutron scattering over a range of supercritical conditions 179 . Even though there seems to be a clear trend that polarizable water models better describe structure under supercritical conditions, they are still under-performing when reproducing thermodynamic properties of the phase equilibria envelope 173,179 .…”