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

Abstract

Using density functional theory (DFT) and density functional perturbation theory (DFPT) with van der Waals (vdW) functionals, this study examined the structural phase behavior of solid molecular hydrogen across pressures from 200 to 450 GPa. Five crystal structures—C2/c, Cmca-12, P6/m, Cmca, and Pbcn—were evaluated through enthalpy and Gibbs free energy calculations to construct zero- and finite-temperature phase diagrams. The C2/c and Pbcn structures were identified as candidates for phase III, each stabilized in distinct pressure sub-ranges above 200 GPa. The choice of vdW functional was found to substantially alter phonon spectra and finite-temperature phase stability, with different functionals yielding divergent results. Beyond vdW interactions, accurate treatment of the high charge gradient limit proved critical. Exchange-correlation dependence of molecular bond lengths introduced errors of up to 100 GPa in the predicted metallization pressure.

Mechanism

The choice of vdW functional alters phonon spectra and finite-temperature phase stability in solid hydrogen; exchange-correlation treatment of molecular bond lengths introduces errors up to 100 GPa in calculated metallization pressure.