  Molecular hydrogen adsorption in a nanoporous metal organic framework
structure (MOF-74) was studied via van der Waals density-functional
calculations. The primary and secondary binding sites for H$_2$ were confirmed.
The low-lying rotational and translational energy levels were 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 H$_2$ loadings and those
measured by inelastic neutron scattering exciting the singlet to triplet (para
to ortho) transition in H$_2$. The H$_2$ binding energy after zero point energy
correction due to the rotational and translational motions is predicted to be
$\sim$100 meV in good agreement with the experimental value of $\sim$90 meV.
