To investigate the contribution of geometry on the adsorption process, we present a theoretical study of the low-pressure physisorption of hydrogen on isolated nanotubes and nanotube bundles through the second virial coefficient, B-AS, computed classically with an uncorrugated adsorption potential. The optimal nanotube bundle geometry at low pressure for a Lennard-Jones adsorption potential is obtained by studying the second virial coefficient, B-AS, for variable radius or bundle lattice constant. The most favorable bundle adsorption sites at low pressures and temperatures are identified for typical bundle structures and the relative contribution of interstitial sites relative to other sites is discussed as a function of temperature and pressure. The Boyle temperature behavior for the B-AS virial coefficient is also discussed as a function of radius for isolated nanotubes. For a given nanostructure, the maximum pressure of applicability of the B-AS approach, below which the adsorption isotherm is linear, is estimated as a criterion which depends on temperature.