  The hydrogen-methane compound (H_2)_4CH_4---or for short H4M---is one of the
most promising hydrogen-storage materials. This van der Waals compound is
extremely rich in molecular hydrogen: 33.3 mass%, not including the hydrogen
bound in CH_4; including it, we reach even 50.2 mass%. Unfortunately, H4M is
not stable under ambient pressure and temperature, requiring either low
temperature or high pressure. In this paper, we investigate the properties and
structure of the molecular and crystalline forms of H4M, using ab initio
methods based on van der Waals DFT (vdW-DF). We further investigate the
possibility of creating the pressures required to stabilize H4M through
external agents such as metal organic framework (MOF) materials and carbon
nanotubes, with very encouraging results. In particular, we find that certain
MOFs can create considerable pressure for H4M in their cavities, but not enough
to stabilize it at room temperature, and moderate cooling is still necessary.
On the other hand, we find that all investigated carbon nanotubes can create
the high pressures required for H4M to be stable at room temperature, with
direct implications for new and exciting hydrogen-storage applications.
