Hydrogen Bonding and Dipole Moment of Water at Supercritical Conditions: A First-Principles Molecular Dynamics Study

We present a first-principles molecular dynamics study of water near and above the critical point ( $T=647\mathrm{}\mathrm{K}$, $\rho =0.32\mathrm{}\mathrm{g}/{\mathrm{cm}}^3$). We find that the systems undergo fast dynamics with continuous formation and breaking of H bonds. At low density, the system fragments mostly into trimers, dimers, and single molecules. At a higher density, more complex structures appear and an extended, albeit very dynamical, H-bond network can be identified. These structures have important consequences for the screening properties of the system. This offers a clue to understanding the peculiar chemical behavior of a supercritical system and allows thermodynamical tuning of its solvent properties.