Quantum Nuclear Motion of Helium and Molecular Nitrogen Clusters in Carbon Nanotubes

We study the quantum nuclear motion of N ⁴He atoms or N N₂ molecules (N < 4) confined in carbon nanotubes using an ad hoc-developed nuclear wave function-based approach. Density functional theory (DFT)-based symmetry-adapted perturbation theory is used to derive parameters for a new pairwise potential model describing the gas adsorption to carbon materials. The predicted nuclear motion of He atoms inside a confining potential is directly compared to probability densities obtained by orbital-free He-DFT theory. The interaction of small clusters of adsorbates is also studied via a combination of both the discrete atomic and the continuous density approaches. Our results agree well with previously reported experimental and theoretical studies and provide new physical insights into the very different quantum confinement effects depending on the diameter of the carbon nanotubes and the impact of quantum phenomena on the gas storage capabilities at low temperatures.