First principles simulations of dense hydrogen

TL;DR

This paper reviews recent first-principles simulations of dense hydrogen, benchmarking various methods, updating the phase diagram, and discussing thermodynamic and dynamic properties relevant to astrophysics and laboratory experiments.

Contribution

It provides a comprehensive benchmark of recent PIMC and DFT results, updates the high-pressure hydrogen phase diagram, and discusses strategies for combining simulation methods for improved accuracy.

Findings

Updated hydrogen phase diagram at high pressures

Benchmarking of PIMC and DFT simulation results

Analysis of thermodynamic and dynamic properties of warm dense hydrogen

Abstract

Accurate knowledge of the properties of hydrogen at high compression is crucial for astrophysics (e.g. planetary and stellar interiors, brown dwarfs, atmosphere of compact stars) and laboratory experiments, including inertial confinement fusion. There exists experimental data for the equation of state, conductivity, and Thomson scattering spectra. However, the analysis of the measurements at extreme pressures and temperatures typically involves additional model assumptions, which makes it difficult to assess the accuracy of the experimental data. rigorously. On the other hand, theory and modeling have produced extensive collections of data. They originate from a very large variety of models and simulations including path integral Monte Carlo (PIMC) simulations, density functional theory (DFT), chemical models, machine-learned models, and combinations thereof. At the same time, each of these methods has fundamental limitations (fermion sign problem in PIMC, approximate exchange-correlation functionals of DFT, inconsistent interaction energy contributions in chemical models, etc.), so for some parameter ranges accurate predictions are difficult. Recently, a number of breakthroughs in first principle PIMC and DFT simulations were achieved which are discussed in this review. Here we use these results to benchmark different simulation methods. We present an update of the hydrogen phase diagram at high pressures, the expected phase transitions, and thermodynamic properties including the equation of state and momentum distribution. Furthermore, we discuss available dynamic results for warm dense hydrogen, including the conductivity, dynamic structure factor, plasmon dispersion, imaginary-time structure, and density response functions. We conclude by outlining strategies to combine different simulations to achieve accurate theoretical predictions.