Supercritical water at ten densities from 0.1 gr/cc to 1.0 gr/cc at 1,000K using quantum molecular dynamics simulations

Supercritical water, found in extreme environments such as Earth's mantle and widely used in advanced chemical and energy technologies, exhibits remarkable departures from the familiar behavior of ambient water. Above its critical point at 647 K and 221 bar, water undergoes a dramatic transformation: the dielectric constant collapses, molecular reactivity increases, and the conventional tetrahedral hydrogen bond network is damaged.

In this talk, I will present a quantum molecular dynamics study of supercritical water at 1000 K across ten densities ranging from 0.1 to 1.0 g/cc Using density functional theory with the SCAN exchange correlation functional, we characterize the evolving structure and dynamics of water under supercritical conditions through atom resolved radial distribution functions, neutron and X ray structure factors, and bond angle distributions. To probe the dynamical response, we analyze velocity and current autocorrelation functions, along with their Fourier transforms, the vibrational density of states and infrared spectra, obtained directly from first principles trajectories.

A central component of this work is a rigorous and electronically grounded definition of hydrogen bonding based on charge density overlap, which remains reliable even in the extreme thermodynamic regime of supercritical water. This metric reveals that although the ambient tetrahedral network is fully disrupted, residual and highly transient hydrogen bonds persist across all densities studied, giving rise to a refined and quantitative picture of the molecular organization and dynamical behavior of supercritical water.

Neural network quantum molecular dynamics simulations will also be discussed.