The role of van der Waals and exchange interactions in high-pressure solid hydrogen

TL;DR

This study explores how van der Waals and exchange interactions influence the phase stability and metallization pressure of high-pressure solid hydrogen, emphasizing the importance of accurate functional choice in density functional theory calculations.

Contribution

It provides a detailed analysis of the effects of vdW functionals and exchange-correlation on phase diagrams and metallization pressure in solid hydrogen, proposing new stable structures for phase III.

Findings

vdW functionals significantly affect vibrational properties and phase stability

Different vdW functionals have varying impacts on results

Incorrect treatment of charge gradients can lead to large errors in metallization pressure

Abstract

We investigate the van der Waals interactions in solid molecular hydrogen structures. We calculate enthalpy and the Gibbs free energy to obtain zero and finite temperature phase diagrams, respectively. We employ density functional theory (DFT) to calculate the electronic structure and Density functional perturbation theory (DFPT) with van der Waals (vdW) functionals to obtain phonon spectra. We focus on the solid molecular $C2/c$, $Cmca$-12, $P6_3/m$, $Cmca$, and $Pbcn$ structures within the pressure range of 200 $<$ P $<$ 450 GPa. We propose two structures of the $C2/c$ and $Pbcn$ for phase III which are stabilized within different pressure range above 200 GPa. We find that vdW functionals have a big effect on vibrations and finite-temperature phase stability, however, different vdW functionals have different effects. We conclude that, in addition to the vdW interaction, a correct treatment of the high charge gradient limit is essential. We show that the dependence of molecular bond-lengths on exchange-correlation also has a considerable influence on the calculated metallization pressure, introducing errors of up to 100GPa.