Bound-State Breaking and the Importance of Thermal Exchange–Correlation Effects in Warm Dense Hydrogen

Abstract

Hydrogen at extreme temperatures and pressures is of key relevance for cutting-edge technological applications, with inertial confinement fusion research being a prime example. In addition, it is ubiquitous throughout our universe and naturally occurs in a variety of astrophysical objects. In the present work, we present exact 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.

Ab Initio Path Integral Monte Carlo Simulations of the Uniform Electron Gas on Large Length Scales

“…More importantly, the ξ extrapolation method will allow for the first time the study of long-range phenomena in real WDM systems based on ab initio PIMC simulations starting with hydrogen. − This will facilitate the interpretation of XRTS experiments in a forward scattering geometry, where the system is effectively probed on large length scales. From a physical perspective, one can expect that such measurements will be highly sensitive to the density of the probed system and, in this way, will complement backward scattering measurements that are particularly sensitive to parameters such as the temperature and ionization. − In accordance with this reasoning, a second potential application of the ξ extrapolation method that is related to the study of WDM concerns the estimation of thermodynamic quantities and transport coefficients such as thermal conductivity and electrical conductivity, which may substantially depend on exchange–correlation effects …”