  The equation of state of H2 adsorbed in the interstitial channels of a carbon
nanotube bundle has been calculated using the diffusion Monte Carlo method. The
possibility of a lattice dilation, induced by H2 adsorption, has been analyzed
by modeling the cohesion energy of the bundle. The influence of factors like
the interatomic potentials, the nanotube radius and the geometry of the channel
on the bundle swelling is systematically analyzed. The most critical input is
proved to be the C-H2 potential. Using the same model than in planar graphite,
which is expected to be also accurate in nanotubes, the dilation is observed to
be smaller than in previous estimations or even inexistent. H2 is highly
unidimensional near the equilibrium density, the radial degree of freedom
appearing progressively at higher densities.
