Supercritical water at ten densities from 0.1 to 1.0 gr/cc at 1000 K using <i>ab initio</i> molecular dynamics simulations

Abstract

Supercritical water is found inside Earth’s mantle, where water is subjected to very high temperatures and pressures. It exhibits extraordinary properties, such as having a low dielectric constant and high reactivity, which stems from the breakdown of the hydrogen bond network in a supercritical state. This makes supercritical water a non-polar solvent and the basis for many innovative technologies. We investigate supercritical water at ten densities (0.1–1.0 gr/cc) at 1000 K to study the structural correlations, such as atom-resolved partial pair distributions, co-ordination numbers, bond-angle distributions and neutron scattering, and x-ray structure factors. Among the dynamical correlations, we investigate the velocity autocorrelation function, current–current correlation function, and their Fourier transforms—vibrational density-of-states and frequency dependent dielectric constant. Structural and dynamical correlations are computed from time-trajectories of the positions and velocities calculated ab initio molecular dynamics within the density functional theory framework using the SCAN exchange–correlation functional. Our results for structural correlations are compared with the neutron scattering experiments on supercritical water by Soper and collaborators [J. Chem. Phys. 106, 247–254 (1997)] and dynamical correlations in the supercritical state are compared with the inelastic neutron scattering results by Car and collaborators [J. Phys. Chem. Lett. 11, 9461–9467 (2020)].