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.
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