We have used molecular dynamics simulations to investigate interaction of H 2 molecules on the exterior surfaces of carbon nanotubes ͑CNTs͒: single and bundle types. At 80 K and 10 MPa, it is found that charge transfer occurs from a low curvature region to a high curvature region of the deformed CNT bundle, which develops charge polarization only on the deformed structure. The long-range electrostatic interactions of polarized charges on the deformed CNT bundle with hydrogen molecules are observed to induce a high local-ordering of H 2 gas that results in hydrogen liquefaction. Our predicted heat of hydrogen liquefaction on the CNT bundle is 97.6 kcal kg −1 . On the other hand, hydrogen liquefaction is not observed in the CNT of a single type. This is because charge polarization is not developed on the single CNT as it is symmetrically deformed under the same pressure. Consequently, the hydrogen storage capacity on the CNT bundle is much higher due to liquefaction than that on the single CNT. Additionally, our results indicate that it would also be possible to liquefy H 2 gas on a more strongly polarized CNT bundle at temperatures higher than 80 K. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1929084͔In 1999, Ye et al. 1 first reported an experimental finding that bundles of single-walled carbon nanotubes ͑SWCNTs͒ could absorb hydrogen in excess of 8 wt % at 100 bar and 80 K. This result satisfies the target of the Department of Energy: namely that hydrogen fuel cell cars require a hydrogen capacity of 6.5 wt % to match the range of a gasolinepowered car. Ye et al. 1 suggested that when individual SWCNTs are separated at a pressure higher than 40 bar hydrogen is physisorbed on the exposed surfaces of the SWCNTs. We now report another conclusion. In general, gas molecules form a monolayer surface coverage on solids, 2 as proved by the well-known Brunauer-Emmett-Teller ͑BET͒ theory of adsorption. 3 When a surface is completely covered with a monolayer of adsorbate, additional molecules are adsorbed on the first layer of the adsorbate. However, the van der Waals interaction energy of the solid surface with the additional layers is negligible in comparison with the interaction energy of the molecules of the first layer, and it is therefore difficult for additional layers to form. Consequently, this BET theory indicates that the physisorption capacity of hydrogen at a constant temperature has no linear relationship with the gas pressure. 4 In other words, the hydrogen physisorption capacity initially increases as the gas pressure increases, but then decreases. This phenomenon is called supercritical adsorption. 4 However, Ye et al. 1 experimentally showed that the hydrogen uptake capacity increases almost linearly with the increased hydrogen pressures. In addition, Gao et al. 5 reported that a single CNT with a cap shows a H 2 uptake capacity of less than 2.18 wt % hydrogen at 77 K and 10 MPa. This begs the question of why a high H 2 uptake capacity can only be found in CNT bundles. We therefore examine how...