Dilation-Induced Phases of Gases Absorbed within a Bundle of Carbon Nanotubes

A study is presented of the effects of gas (especially ${\mathrm{H}}_2$) absorption within the interstitial channels of a bundle of carbon nanotubes. The ground state of the system is determined by minimizing the total energy, which includes the molecules' interaction with the tubes, the intertube interaction, and the molecules' mutual interaction (which is screened by the tubes). The consequences of swelling include a reduced threshold pressure for gas uptake and a $2.7\%$ increase in the tubes' breathing mode frequency.