Simulating Raman Spectra using molecular dynamics, and identification of high-pressure phases III and IV in hydrogen

TL;DR

This paper introduces a method to extract Raman spectra from ab initio molecular dynamics simulations, successfully identifying high-pressure phases of hydrogen and distinguishing them from previously proposed structures.

Contribution

The authors develop a novel technique for calculating Raman intensities from MD simulations and apply it to high-pressure hydrogen, revealing new phase structures and clarifying phase boundaries.

Findings

Identified phase III - IV boundary at high temperature and pressure.

Discovered a dynamic hexagonal structure for high-temperature phase IV.

Showed previous phase IV structures are inconsistent with experimental data.

Abstract

We present a technique for extracting Raman intensities from ab initio molecular dynamics (MD) simulations at high temperature. The method is applied to the highly anharmonic case of dense hydrogen up to 500 K for pressures ranging from 180 GPa to 300 GPa. On heating or pressurizing we find first-order phase transitions at the experimental conditions of the phase III - IV boundary. Direct comparison of Raman vibrons with experiment provides excellent discrimination between subtly different structures, found in MD. We find candidate structures whose Raman spectra are in good agreement with experiment. The new phase obtained in high temperature simulations adopts a dynamic, simple hexagonal structure with three layer types: freely rotating hydrogen molecules, static hexagonal trimers and rotating hexagonal trimers. We show that previously calculated structures for phase IV are inconsistent with experiment, and their appearance in simulation is due to finite size effects.