for the protonated molecule only (Figure ), with the carboxylic acid groups left free to rotate (full line) or fixed in a plane (dashed line). The latter situation mimicks a strongly directional bond between the carboxylic acid group and the metal cluster in the MOF; the real system probably provides a less rigid constraint on the orientation. A polynomial fit to the calculated energies shows that the minimum energy is found at an angle of approximately 30 in reasonable agreement with the 45 previously obtained with the Hartree-Fock method and the 30 obtained with hybrid DFT. The difference in energy between the minima of the double well and the transition state at 0 is calculated to be 52 meV and 30 meV for the free and constrained carboxylic acid groups, respectively, corresponding well with the values calculated from the MD simulations and previous calculations of the rotational barrier in biphenyl. As hybrid functionals are known to provide a better description of molecular geometries the present calculations were also performed with the PBE0 functional, which showed qualitatively similar results but slightly larger values for the energy barrier (39-77 meV) and the optimal torsion angle (28 -38 , c.f. ESI).