1.5 CO / f.u. ). In the case of CO2 on top of the metal site, we find no mixing of the metal-molecule orbitals, as expected from an electrostatic/dispersion interaction (see PDOS in Figure ). The strength of the π-acceptor interaction was computed by performing a Bader charge analysis using PAW pseudopotentials and the PBE+D3+BJ functional. For the CO on site A, the analysis revealed a small charge transfer from the MOF to the molecule of 0.05 electrons, confirming a . Energy shift with respect to the bare material of the 6 most characteristic modes plotted versus the van der Waals volume occupied by the guest molecules. We report the mode at 107.9 cm-1 (purple data, upper panel), 137.8 (black), 138.5 (light blue), and 139.2 cm -1 (orange, middle panel), 403.6 (green) and 405.0 cm -1 (red, lower panel). The points in the abscissa correspond to a=16.2 (1 CO/f.u.), b=20.2 (1 CO 2 /f.u.), c=23.8 (1 SO 2 / f.u.), d=24.2 (1.5 CO/f.u.), e=30.3 (1.5 CO 2 / f.u.), and f=32.4 (2 CO/f.u.) cm 3 /mol.