  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 in 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).
