Crossover of dynamical instability and chaos in the supercritical state

TL;DR

This study investigates how the maximal Lyapunov exponent changes across the supercritical state in Lennard-Jones particles, revealing a crossover at the Frenkel line that links liquid-like diffusion and gas-like collisions.

Contribution

It demonstrates a connection between dynamical instability measures and the Frenkel line in supercritical fluids, providing new insights into phase space dynamics far from melting conditions.

Findings

Lyapunov exponent exhibits a crossover at the Frenkel line.

Crossover corresponds to a shift from diffusion to collision-dominated dynamics.

Results apply to supercritical states beyond the applicability of rare-gas models.

Abstract

We calculate the maximal Lyapunov exponent for a bulk system of 256 Lennard-Jones particles in constant energy molecular dynamics simulations deep into the supercritical state. We find that the maximal Lyapunov exponent undergoes a crossover, and that this crossover coincides with the dynamical crossover at the Frenkel line from liquid physics. We explain this crossover in terms of two different contributions to dynamical instability: diffusion in the liquid-like state below the Frenkel line, and collisions in the gas-like state above. These results provide insight into the phase space dynamics far from the melting line and densities where rare-gas approximation are inapplicable.