Colloquium: Structure search for the H2-PRE phase of solid hydrogen

ABSTRACT

Hydrogen crystallizes when pressure is applied and is expected to metallize at high-pressure limit. By experimentally capturing discontinuous behaviors such as in Raman frequencies, several different crystalline phases are known to exist, as summarized in a phase diagram. However, with the exception of the crystalline phase appeared in the lower pressure range, its crystal structure has not been experimentally identified. To clarify this, simulation research has been developed to predict the crystal structures. Initiated with the application of ab initio molecular dynamics, a number of crystal structures have been predicted by methods such as random search using density functional theory. However, the predictions are known to be sensitive to the reliability of each approximation level of adopted simulation methods. As such, the most reliable ab initio quantum Monte Carlo method has been the minimum standard level for this problem. Recently, it has been found that a crystalline phase called 'PRE phase' exists in the pressure region just before entering the metallic phase. We have successfully predicted the crystal structure realized there. We first narrowed down the candidate crystal structures to the density functional theory level using a particle swarm algorithm. We then used ab initio quantum Monte Carlo methods to further refine the search by accurately evaluating electron-electron interactions. Careful consideration was given to phonon contributions to determine the final candidate structure.

Phys. Rev. B 104, 214111 (2021) 

https://doi.org/10.1103/PhysRevB.104.214111

BIOGRAPHY

Prof. Maezono is a Professor at JAIST, working on Simulation Science. He got his BSc and PhD at Tokyo Univ. (Applied Physics/condensed matter theory on phase diagrams of magnetic oxides). He was a postdoc at Cavendish Lab., Cambridge Univ. (EPSRC fellow/2000-2002), and moved to NIMS (National Institute of Materials Science, Japan), as a tenure researcher (2001-2007), then moved to JAIST. Since his postdoc in Cambridge, he has worked on ab initio Diffusion Monte Carlo (DMC) calculations using massive parallel computations. As an expert of DMC method, he has given several lectures on many-body problems at several external National Universities in Japan. As a computer scientist, he has contributed also to the education of simulation, which contents are published in his books (ISBN:978-4627818217, 978-4627170315). He also leads several industrial collaborations with companies (Toyota-Motor/Sumitomo-Mining etc.), as well as those with experimental synthesis community in inorganic Chemistry.