[MOF+CO2] ensemble and the separated components; a negative value indicates a favorable interaction, while the opposite holds for positive ΔEads values. The most favorable adsorption site is the tetrahedral pore, even if it is slightly smaller than the octahedral one. This energy stabilization comes from a hydrogen bond interaction between CO2 and a hydroxyl group on the inorganic cluster (Figure ), which is known to be the most favorable adsorption site for polar and quadrupolar species in UiO-66. If the (ΔEads)T values found for the different materials are compared, an increase in the interaction energy is found when moving from MOF-801 to ZrTFS, in perfect agreement with the Qst trend reported in Table . It is to be noted that the window site in ZrTFS is associated with a positive (ΔEads) value, which suggests that CO2 diffusion through the windows is unfavourable, explaining the low adsorption capacity of this MOF.