Bound state breaking and the importance of thermal exchange-correlation effects in warm dense hydrogen

TL;DR

This paper presents exact ab initio PIMC results for warm dense hydrogen, introduces a new framework to identify bound state breaking, and assesses the accuracy of exchange-correlation functionals, highlighting the importance of thermal XC effects.

Contribution

It provides a new framework using reduced density gradients to identify bound state breaking and offers a rigorous assessment of XC functionals against PIMC data.

Findings

Bound states break due to pressure ionization at high densities.

Thermal XC effects are crucial for accurate density gradient descriptions.

The PIMC results serve as a benchmark for future simulations.

Abstract

Hydrogen at extreme temperatures and pressures is ubiquitous throughout our universe and naturally occurs in a variety of astrophysical objects. In addition, it is of key relevance for cutting-edge technological applications, with inertial confinement fusion research being a prime example. In the present work, we present exact \emph{ab initio} path integral Monte Carlo (PIMC) results for the electronic density of warm dense hydrogen along a line of constant degeneracy across a broad range of densities. Using the well-known concept of reduced density gradients, we develop a new framework to identify the breaking of bound states due to pressure ionization in bulk hydrogen. Moreover, we use our PIMC results as a reference to rigorously assess the accuracy of a variety of exchange--correlation (XC) functionals in density functional theory calculations for different density regions. Here a key finding is the importance of thermal XC effects for the accurate description of density gradients in high-energy density systems. Our exact PIMC test set is freely available online and can be used to guide the development of new methodologies for the simulation of warm dense matter and beyond.