We have studied, by first-principles methods, the interaction of molecular hydrogen with a double-walled (2,10) carbon nanotube
(DWCNT). This combination of the smallest possible diameter for the inner nanotube with a significantly larger outer tube
allows for substantial space between the nanotube walls, in which molecular hydrogen can adsorb. We performed classical force
field molecular dynamics simulations of the infinitely extended (periodic) DWCNT with varying amounts of hydrogen, which showed
that, depending on the H-2 loading, both a coaxial and a noncoaxial DWCNT configuration can be stable. We then carried out
the electronic structure calculations on both the coaxial and the noncowdal geometries of the nanotubes to accurately compute
the H-2 adsorption pathways inside the DWCNT. Interestingly, the noncoaxial DWCNT (2,10) shows a barrierless reaction path
to dissociate the H-2 molecule. We also investigated the case of a DWCNT of finite length, whose edges are either clean or
terminated with H atoms, and we searched for the favorite adsorption sites for a H-2 molecule in the interstitial region between
the inner and the outer tube. The finite DWCNT whose edges are passivated by H atoms can be suggested as a potential candidate
for hydrogen storage. The H-2 molecules, in fact, may enter in the cavity between the two nanotubes without reacting with
the dangling bonds of the C atoms and can be physisorbed with a binding energy of about 0.06 eV, suitable for hydrogen storage.
We emphasize the important role played in the physisorbed states for all the systems studied by the van der Waals interactions,
which are properly included in the present study. Compared to the interaction of H-2 with graphene and the single-walled carbon
nanotube (SWCNT), the existence of another carbon layer, for the coaxial DWCNT, does not significantly lower the energy barriers
for chemisorption and instead enhances the binding energy of the H-2 molecule to the inner tube up to 0.1 eV.