-Quantum Effects or Theoretical Artifacts? A Computational Reanalysis of Hydrogen's High-Pressure Phase Stability and Properties

Abstract

Experimental and theoretical studies of the high-pressure phases of hydrogen are highly sensitive to methodological choices. We compare the cold phase diagram of hydrogen calculated between 400-700 GPa with meta-GGA functionals (R2SCAN and SCAN0) with the commonly employed GGA-PBE functional. Molecular phases are stabilized over the atomic phase to higher pressures with the meta-GGA, in closer agreement with diffusion Monte Carlo calculations. R2SCAN phonon spectra show that the dynamical instabilities and anharmonic signatures predicted at the GGA level vanish, indicating that such effects may partly arise from functional deficiencies rather than quantum nuclear effects. Bonding analysis reveals that PBE artificially weakens intramolecular H-H bonds and enhances intermolecular interactions through charge delocalization, whereas meta-GGA preserves a more localized molecular character.