Effect of amount of adsorbed gas on the spectral features. In Figure we show a comparison of the computed G(ω) for 1, 1.5 and 2 CO adsorbed molecules with the experimental INS data (see Figure ). In the presence of a single molecule, it can be observed that neither of the two characteristic signatures of occurs. The peaks at around 108, 138 and 400 cm -1 do not undergo any displacement, whereas in the other two cases, both peaks are shifted upon adsorption. We performed a similar comparison for 1 and 1.5 CO2 molecules adsorbed per f.u. (see Figure ). In an attempt to differentiate the steric effect from the nature of the binding, we report the shift of the 6 most characteristic and intense modes as a function of the volume occupied by the adsorbed molecules. The computed van der Waals volume is considered in each case. Specifically, we report the shift upon adsorption for the mode at 107.9 cm -1 (upper panel), for the three modes under peak A (middle panel) and for the two modes dominating the intensity of peak D (lower panel) in Figure .