Density functional theory has been used to study the interaction of molecular and atomic hydrogen with ͑5,5͒ and ͑6,6͒ single-wall carbon nanotubes. Static calculations allowing for different degrees of structural relaxation are performed, in addition to dynamical simulations. Molecular physisorption inside and outside the nanotube walls is predicted to be the most stable state of those systems. The binding energies for physisorption of the H 2 molecule outside the nanotube are in the range 0.04-0.07 eV. This means that uptake and release of molecular hydrogen from nanotubes is a relatively easy process, as many experiments have proved. A chemisorption state, with the molecule dissociated and the two hydrogen atoms bonded to neighbor carbon atoms, has also been found. However, reaching this dissociative chemisorption state for an incoming molecule, or starting from the physisorbed molecule, is difficult because of the existence of a substantial activation barrier. The dissociative chemisorption deforms the tube and weakens the CuC bond. This effect can catalyze the shattering and scission of the tube by incoming hydrogen molecules with sufficient kinetic energy.

Density functional theory has been used to study the adsorption of molecular H 2 on a graphene layer. Different adsorption sites on top of atoms, bonds and the center of carbon hexagons have been considered and compared. We conclude that the most stable configuration of H 2 is physisorbed above the center of an hexagon. Barriers for classical diffusion are, however, very small.

The H interaction with the Pd dimer and trimer were studied using multiconfigurational 2 Ž . self-consistent field MC-SCF calculations with the relativistic effective core potential Ž . RECP ; the correlation energy correction was included in the extended multireference Ž . configuration interaction MRCI , variational and perturbative to second order. Here, we considered the Pd first six states:

Density functional calculations are performed to study the interaction of molecular and atomic hydrogen with (5 5) and (6 6) single-wall carbon nanotubes. Molecular physisorption is predicted to be the most stable adsorption state, with the molecule at equilibrium at a distance of 5-6 a.u. from the nanotube wall. The physisorption energies outside the nanotobes are approximately 0.07 eV, and larger inside, reaching a value of 0.17 eV inside the (5 5) nanotube. Although these binding energies appear to be lower than the values required for an efficient adsorption/desorption operation at room temperature and normal pressures, the expectations are better for operation at lower temperatures and higher pressures, as found in many experimental studies. A chemisorption state with the molecule dissociated has also been found, with the H atoms much closer to the nanotube wall. However, this state is separated from the physisorption state by an activation barrier of 2 eV or more. The dissociative chemisorption weakens carbon-carbon bonds, and the concerted effect of many incoming molecules with sufficient kinetic energies can lead to the scission of the nanotube.

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