Molecular hydrogen adsorption in a nanoporous metal-organic framework structure (MOF-74) is studied via van der Waals density-functional calculations. The primary and secondary binding sites for ${\mathrm{H}}_2$ are confirmed. The low-lying rotational and translational energy levels are calculated, based on the orientation and position dependent potential energy surface at the two binding sites. A consistent picture is obtained between the calculated rotational-translational transitions for different ${\mathrm{H}}_2$ loadings and those measured by inelastic neutron scattering exciting the singlet to triplet (para to ortho) transition in ${\mathrm{H}}_2$. The ${\mathrm{H}}_2$ binding energy after zero-point energy correction due to the rotational and translational motions is predicted to be $\sim 100\mathrm{meV}$ in good agreement with the experimental value of $\sim 90\mathrm{meV}$.

• Received 30 June 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.096103