title: On the room-temperature phase diagram of high pressure hydrogen: an ab initio molecular dynamics perspective and a diffusion Monte Carlo study.
abstract: The finite-temperature phase diagram of hydrogen in the region of phase IV and its neighborhood was studied using the ab initio molecular dynamics (MD) and the ab initio path-integral molecular dynamics (PIMD). The electronic structures were analyzed using the density-functional theory (DFT), the random-phase approximation, and the diffusion Monte Carlo (DMC) methods. Taking the state-of-the-art DMC results as benchmark, comparisons of the energy differences between structures generated from the MD and PIMD simulations, with molecular and dissociated hydrogens, respectively, in the weak molecular layers of phase IV, indicate that standard functionals in DFT tend to underestimate the dissociation barrier of the weak molecular layers in this mixed phase. Because of this underestimation, inclusion of the quantum nuclear effects (QNEs) in PIMD using electronic structures generated with these functionals leads to artificially dissociated hydrogen layers in phase IV and an error compensation between the neglect of QNEs and the deficiencies of these functionals in standard ab initio MD simulations exists. This analysis partly rationalizes why earlier ab initio MD simulations complement so well the experimental observations. The temperature and pressure dependencies for the stability of phase IV were also studied in the end and compared with earlier results.
authors.affiliation: International Center for Quantum Materials, Peking University, Beijing 100871, People's Republic of China.
authors.forename: Ji
authors.initials: J
authors.lastname: Chen
authors.affiliation: Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei 230026, Anhui, People's Republic of China.
authors.forename: Xinguo
authors.initials: X
authors.lastname: Ren
authors.affiliation: School of Physics, Peking University, Beijing 100871, People's Republic of China.
authors.forename: Xin-Zheng
authors.initials: XZ
authors.lastname: Li
authors.affiliation: London Centre for Nanotechnology, University College London, London WC1H 0AH, United Kingdom.
authors.forename: Dario
authors.initials: D
authors.lastname: Alfè
authors.affiliation: International Center for Quantum Materials, Peking University, Beijing 100871, People's Republic of China.
authors.forename: Enge
authors.initials: E
authors.lastname: Wang