**6. Conclusion and outlook**

Because of the large number of different MOFs that exist, efforts to predict the performance of MOFs using molecular modeling play an important role in selecting materials for specific applications. The high number of publications on MOFs and the dense interest of academy and industry on these new nanoporous materials hint that MOFs have numerous potential applications. Since almost all of these applications require the knowledge of molecular

Recent Advances in Molecular Dynamics

computationally meaningful time scale.

in MOFs.

**7. References** 

Press,New York

*Ed.*, 46, 463-466.

6840.

*Am. Chem. Soc.*, 131, 8401-8403.

Simulations of Gas Diffusion in Metal Organic Frameworks 273

challenges in using MD simulations for MOFs was addressed in Section 2.2: absence of fully flexible force fields. Rigid framework assumption creates tremendous savings in computational effort. A handful of studies used flexible force fields to include the lattice dynamics effects on gas diffusivity in MOFs. These studies showed that there can be orders of magnitude difference between the diffusivity data from MD simulations using rigid framework and the one using flexible framework, specifically for large adsorbates. This issue indeed turns to be related with the challenge listed above, having accurate flexible interatomic potentials which can be applied to a family of MOF structures in a

Another major challenge, especially in diffusivity simulations of CO2 and N2, is the choice of method to assign partial charges to MOF atoms. The QM calculations were used to define partial point charges in the literature, however there is no unique way to accomplish this task and different charge decomposition methods can give rather different results. Studies have shown that charge effects are important especially for computing diffusivities at low loadings. Careful studies that establish reliable approaches in charge assignment will be very useful in employing MD simulations for diffusion of polar and quadrupolar molecules

To date MD simulations have been used to compute the transport rates of adsorbates in MOFs. One remaining challenge is to predict the long term stability of MOFs since this is a serious issue in practical applications of these materials. Although stability issue sounds to be most likely addressed by experimental studies, one recent MD study which investigated the mechanism of water induced decomposition of IRMOF-1(Greathouse&Allendorf, 2006)

Ackerman, D. M., Skoulidas, A. I., Sholl, D. S.&Johnson, J. K. (2003). Diffusivities of Ar and

Allen, M. P.&Tildesley, D. J. (1987). *Computer Simulation of Liquids*. Oxford University

Amirjalayer, S., Tafipolsky, M.&Schmid, R. (2007). Molecular Dynamics Simulation of

Amirjalayer, S.&Schmid, R. (2009). Mechanism of Benzene Diffusion in MOF-5: A Molecular

An, J., Geib, S.&Rosi, N. L. (2009a). Cation-Triggered Drug Release from a Porous Zinc-Adeninate Metal-Organic Framework. *J. Am. Chem. Soc.*, 131, 8376-8377. An, J., Fiorella, R., Geib, S. J.&Rosi, N. L. (2009b). Synthesis, Structure, Assembly, and

Atci, E., Erucar, I.&Keskin, S. (2011). Adsorption and Transport of CH4, CO2, H2 Mixtures

Dynamics Investigation. *Micropor. Mesopor. Mater.*, 125, 90-96.

Benzene Diffusion in MOF-5: Importance of Lattice Dynamics. *Angew. Chem. Int.* 

Modulation of the CO2 Adsorption Properties of a Zinc-Adeninate Macrocycle. *J.* 

in a Bio-MOF Material from Molecular Simulations. *J. Phys. Chem. C*, 115 6833-

showed that molecular simulations can be also helpful in this area.

Ne in Carbon Nanotubes. *Mol. Simulat.*, 29, 677-684.

transport rates, MD studies become one of the most beneficial methods in studying MOFs. As is evident from the volume of literature cited, this area is growing rapidly. The development of quantitative information about mixture diffusion in MOFs is just beginning (section 3.2) whereas a significant number of studies have already considered single component gas diffusion in MOFs (section 3.1). Detailed understanding of mixture diffusion in MOFs will be very beneficial for design of MOF membranes, adsorbents, catalysts and sensors. Current opportunities and challenges of using MD simulations in assessing transport rates of gases in MOFs will be addressed below.
