We perform the density-functional and quantum chemical calculations of adso
rption of a hydrogen molecule to various kinds of single-walled carbon nano
tubes (SWNT's). The potential energy barrier height (PBH) for the dissociat
ive adsorption of a hydrogen molecule onto the outer wall of a nanotube dec
reases as the tube diameter decreases. In contrast, the PBH for H-2 penetra
tion into an open-ended nanotube increases as the tube diameter decreases,
independent of the atomic geometry, i.e., zigzag or armchair structures. H-
2, however, cannot adsorb to the inner wall of any type of nanotube. These
results on the structure-dependent H-2 adsorption to SWNT's are reasonably
explained by the sp(2)-sp(3) rehybridization caused by the interaction betw
een the adsorbing hydrogen and carbon atoms of SWNT's.