Quantum molecular dynamics simulations for the nonmetal-metal transition in shocked methane

TL;DR

This study uses quantum molecular dynamics to investigate the nonmetal-metal transition in shocked methane up to 80 GPa, revealing the transition's relation to dissociation and challenging previous assumptions about diamond-like configurations.

Contribution

First quantum molecular dynamics simulation of shocked methane showing the transition to metallic state linked to dissociation, without diamond-like structures.

Findings

Methane becomes metallic near dissociation densities.

No diamond-like configurations observed at studied conditions.

Optical conductivity and electronic structure analyses support the transition.

Abstract

We have performed quantum molecular-dynamics simulations for methane under shock compressions up to 80 GPa. We obtain good agreement with available experimental data for the principal Hugoniot, derived from the equation of state. A systematic study of the optical conductivity spectra, one-particle density of states, and the distributions of the electronic charge over supercell at Hugoniot points shows that the transition of shocked methane to a metallic state takes place close to the density at which methane dissociates significantly into molecular hydrogen and some long alkane chains. Through analyzing the pair correlation function, we predict the chemical picture of the shocked methane. In contrast to usual assumptions used for high pressure modeling of methane, we find that no diamond-like configurations occurs for the whole density-temperature range studied.