Systematic first-principles molecular dynamics (MD) simulations with long simulation times (7-13 ps) for phase IV of solid hydrogen using different supercell sizes of 96, 288, 576, and 768 atoms established that the diffusive proton motions process i
n the graphene-like layer is an intrinsic property and independent of the simulation cell sizes. The present study highlights an often overlook issue in first-principles calculations that long time MD is essential to achieve ergodicity, which is mandatory for a proper description of dynamics of a system. The present results contradict a recent work [Phys. Rev. B 87, 174110 (2013)] in which the analysis was relied on short time slices (1-3 ps).
