First-Principles Equation of State and Electronic Properties of Warm Dense Oxygen

TL;DR

This study uses first-principles simulations to explore the properties of warm dense oxygen across a wide range of densities and temperatures, providing insights into its equation of state, ionization, and shock response.

Contribution

It combines PIMC and DFT-MD methods to generate a comprehensive equation of state for warm dense oxygen, extending previous models and revealing detailed electronic and structural behavior.

Findings

First-principles pressures and energies match analytic models above 8 million K.

Ionization of the 1s state decreases with density at constant temperature.

Shock Hugoniot curves show increased compression with shell ionization.

Abstract

We perform all-electron path integral Monte Carlo (PIMC) and density functional theory molecular dynamics (DFT-MD) calculations to explore warm dense matter states of oxygen. Our simulations cover a wide density-temperature range of $1-100$~g$\,$cm$^{-3}$ and $10^4-10^9$~K. By combining results from PIMC and DFT-MD, we are able to compute pressures and internal energies from first-principles at all temperatures and provide a coherent equation of state. We compare our first-principles calculations with analytic equations of state, which tend to agree for temperatures above 8$\times$10$^6$~K. Pair-correlation functions and the electronic density of states reveal an evolving plasma structure and ionization process that is driven by temperature and density. As we increase the density at constant temperature, we find that the ionization fraction of the 1s state decreases while the other electronic states move towards the continuum. Finally, the computed shock Hugoniot curves show an increase in compression as the first and second shells are ionized.