Appraisal of the realistic accuracy of Molecular dynamics of high pressure hydrogen

TL;DR

This paper evaluates the accuracy of molecular dynamics simulations for high-pressure hydrogen, highlighting key sources of error and their impact on predicted phase stability, especially the significance of zero-point vibrations.

Contribution

It provides a detailed analysis of error sources in molecular dynamics simulations of high-pressure hydrogen and assesses their effects on phase stability predictions.

Findings

Neglect of zero-point vibrations is the largest error.

Simulations predict a larger stability field for certain phases than observed.

Errors suggest a smaller stability range and possible non-molecular phases.

Abstract

Molecular dynamics is a powerful method for studying the behaviour of materials at high temperature. In practice, however, its effectiveness in representing real systems is limited by the accuracy of the forces, finite size effects, quantization, and equilibration methods. In this paper we report and discuss some calculations carried out using molecular dynamics on high pressure hydrogen, reviewing a number of sources of error. We find that the neglect of zero-point vibrations is quantitively the largest. We show that simulations using ab initio molecular dynamics with the PBE functional predict a large stability field for the molecular Cmca-4 structure at pressures just above those achieved in current experiments above the stability range of the mixed- molecular layered Phase IV. However, the various errors in the simulation all point towards a much smaller stability range, and the likelihood of a non-molecular phase based on low-coordination networks or chains of atoms.