  DFT is a valuable tool for calculating adsorption energies toward designing
materials for hydrogen storage. However, dispersion forces being absent from
the theory, it remains unclear how the consideration of van der Waals (vdW)
interactions affects such calculations. For the first time, we applied
diffusion Monte Carlo (DMC) to evaluate the adsorption characteristics of a
hydrogen molecule on a (5,5) armchair silicon-carbide nanotube (H$_2$-SiCNT).
Within the framework of density functional theory (DFT), we also benchmarked
various exchange-correlation functionals, including those recently developed
for treating dispersion or vdW interactions. We found that the vdW-corrected
DFT methods agree well with DMC, whereas the local (semilocal) functional
significantly over (under)-binds. Furthermore, we fully optimized the
H$_2$-SiCNT geometry within the DFT framework and investigated the correlation
between structure and charge density. The vdW contribution to adsorption was
found to be non-negligible at approximately 1 kcal/mol per hydrogen molecule,
which amounts to 9-29 % of the ideal adsorption energy required for hydrogen
storage applications.
