Hydrogen phase-IV characterization by full account of quantum anharmonicity

TL;DR

This paper introduces a novel computational framework for accurately determining phonons in molecular crystals with strong quantum anharmonicity, validated on high-pressure solid hydrogen, aligning well with experimental vibrational data.

Contribution

The authors develop a low-variance, path integral molecular dynamics-based method to compute phonons in strongly anharmonic molecular crystals, enabling precise vibrational analysis.

Findings

Accurate IR and Raman vibrons for hydrogen phase III match experiments.

Characterization of hydrogen phase IV's symmetry and vibrational peaks.

Efficient phonon computation with short simulation times.

Abstract

We devise a framework to compute accurate phonons in molecular crystals even in case of strong quantum anharmonicity. Our approach is based on the calculation of the static limit of the phononic Matsubara Green's function from path integral molecular dynamics simulations. Our method enjoys a remarkably low variance, which allows one to compute accurate phonon frequencies after a few picoseconds of nuclear dynamics, and it is further stabilized by the use of appropriate constrained displacement operators. We applied it to solid hydrogen at high pressure. For phase III, our predicted infrared (IR) and Raman active vibrons agree very well with experiments. We then characterize the crystalline symmetry of phase IV by direct comparison with vibrational data and we determine the character of its Raman and IR vibron peaks.