To gain insight into the effect of the fluorinated ligand on the MOFs structural properties and CO2 adsorption ability, DFT simulations in the gas phase at the [PBE/MOLOPT] level of theory were carried out. As a first step in the computational analysis, the structures of MOF-801 and ZrTFS were optimized. For MOF-801, the starting geometry was the defect-free crystal lattice with formula [Zr6O4(OH)4(FUM)6], while for ZrTFS the initial guess [Zr6O4(OH)4(TFS)6] was considered. In order to evaluate the effect of the presence of both linkers in the same solid phase on the CO2 adsorption performance from a theoretical viewpoint, a hypothetical (simplified) structure of minimal formula [Zr6O4(OH)4(FUM)4(TFS)2] (PF-MOF) was also built in silico and subsequently optimized. Albeit this computational model cannot be considered fully representative of the real exchanged materials PF-MOF1 and PF-MOF2, it is a useful reference for a comparison between a mixed-linker MOF and its homo-linker counterparts. Indeed, the design of computationally representative defective structures is too challenging, as the linker is placed onto the defects randomly. On these models, a joint variable-cell and atomic positions optimization was performed (see the Computational Details). As shown in Table , the optimized lattice parameters are in perfect agreement with the experimentally determined values, with the reproduction of the slight unit cell size increment moving from pure fumarate to the fluorinated systems. These MOFs feature tetrahedral and octahedral pores interconnected by triangular windows, with a lattice structure similar to that of UiO-66. The windows are smaller than the pores (Figure and Table ) and they could be responsible for the percolation of the gas inside the MOF. The gas adsorption process is driven by two main factors: the host-guest chemical affinity and the (high) accessible SSA. Another interesting factor that deeply influence the adsorption performance is the gas percolation and diffusion inside the material. Literature evidence suggests that MOFs featuring high CO2 chemical affinity but with very small pore windows are poor adsorbents. To compare the window and pores of the different models, the surface of the window is defined as the area of the triangle defined by the three hydrogen (or fluorine) atoms as indicated in Figure , while the pore dimension is evaluated as the distance between opposite hydrogen or fluorine atoms in Figure . The average values of the cavities and windows size are summarized in Table . Moving from MOF-801 to ZrTFS, the octahedral pore size does not change significantly, while the window area and the tetrahedral pore size are considerably reduced. Moreover, the mixed-linker PF-MOF shows variable pore width (between 6 and 9 Å, Table ) that depends on the presence of either TFS 2-or FUM 2-. To gain deeper insights on the preferential CO2 adsorption sites in these materials, CO2 was introduced in the computational model and located in four different lattice positions (Figure ): in the center of the octahedral cavity (1); close to the zirconium ions (2); in the center of the tetrahedral pore (3); and in the middle of the pore windows (4). Afterwards, the ensemble was re-optimized. The CO2 adsorption energy (ΔEads) is then calculated as the energy difference between the optimized