Diffusion and adsorption of aromatic guests in MOFs studied by ab initio and force field simulations including lattice dynamics Louis Vanduyfhuys, Toon Verstraelen, Matthias Vandichel, Jeroen Van der Mynsbrugge, Michel Waroquier, Veronique Van Speybroeck Center for Molecular Modeling, Ghent University, Technologiepark 903, 9052 Zwijnaarde, Belgium Veronique.vanspeybroeck@ugent.be Introduction About a decade ago, a new class of nanoporous molecular systems known as metal- organic frameworks (MOFs) were introduced by the group of Yaghi [1]. One of the most attractive features of MOFs is its very high surface area, e.g. up to 5.640 m 2 /g for MOF177 [2]. Their high surface area makes them a very promising class of materials for hydrogen storage [3]. Other applications can be found in catalysis [4] and gas separation. Recently it was shown that Metal-Organic Frameworks could be successfully used as selective adsorbents for the extremely difficult and industrially relevant separations of mixed C8 alkylaromatic compounds [5,6]. Another remarkable property of MOFs, is its high framework flexibility [7]. The strong interactions between the framework and host molecules, make the MOF prone to drastic volume changes. An example of such a MOF is MIL-53 (Figure 1) for which cases have been reported of volume change as high as 40 % [8]. Another interesting consequence of the framework flexibility is the phenomenon of pore breathing. Due to the interaction with guest molecules, the complete framework is able to expand and shrink. Since MOFs are periodic structures with rather large unit cells (cell parameters are of the order of 10 Å, a full ab initio calculation is readily demanding with the modern computer systems. Therefore force fields are used to perform molecular dynamics simulations. In this study, we wish to develop such a force field for MIL-53 based on