The properties of hydrogen and helium under extreme conditions

Abstract

Hydrogen and helium are the most abundant elements in the universe and, in principle, are the simplest elements. Nonetheless, they display remarkable properties under pressure that have fascinated theoreticians and experimentalists for over a century. Recent advances in computational methods have made it possible to elucidate many of these properties. We review some of the computational methods that have been applied to dense hydrogen and helium in recent years, mainly those that perform a simulation directly from the physical picture of electrons and ions; primarily, those based on density functional theory and quantum Monte Carlo methods. We then discuss the predictions from such methods as applied to the phase diagram of hydrogen, including the solid and fluid phases, with particular focus on the crystal structures, the liquidliquid transition and comparison of the results with experimental shock-wave data. We then discuss predictions of ordered quantum states, including a possible low-temperature fluid and high-temperature superconductivity in the atomic state. We also briefly discuss pure helium, and then focus on hydrogen-helium mixtures, with particular focus on properties of relevance to planetary science.

A solid–solid phase transition in carbon dioxide at high pressures and intermediate temperatures

“…The ab initio computational method reported here, which can be parallel executed routinely on small computer clusters for infinite three-dimensional solids, therefore, holds exceptional promise in assisting experimental studies of molecular crystals, which are important in high-pressure chemistry 2,3,6 , solid-state physics 3,7 , geochemistry 2 , astrophysics 3 , medicinal chemistry 4,5 , supramolecular chemistry 8,9 , materials science 4,7 and more.…”

“…Molecular crystals are chemists' solids, so to speak, as their properties can be understood and even controlled by those of the constituent molecules. They include ices of atmospheric species 2,3 , drugs 4,5 and explosives 6 . They are also related to organic superconductors and other functional materials 7 as well as supramolecular assemblies 8,9 .…”

Elastic anisotropy and Poisson's ratio of solid helium under pressure

“…Many different numerical methods have been applied to solid helium [10][11][12] . The electronic subsystem can either be described from first principles, like in density functional theory (DFT)-based methods, or replaced by empirical pairwise or n-body interactions between nuclei.…”

“…The electronic subsystem can either be described from first principles, like in density functional theory (DFT)-based methods, or replaced by empirical pairwise or n-body interactions between nuclei. The quantum and thermal motion of the nuclei can be analyzed either in harmonic approximation, or using anharmonic approaches, such as diffusion Monte Carlo (DMC) and other Monte Carlo methods 10 .…”

Stable structures of He andH2Oat high pressure

“…1(a) ( stable phases. The Pbcm structure of H 2 O ice and hexagonal close packed structure of He [18] were used as reference structures. The Pbcm ice is the structure predicted to form at about 3 Mbars from ice X [19,20].…”