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

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.

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