Modeling flexibility in metal-organic frameworks: Comparison between Density-Functional Tight-Binding and Universal Force Field approaches for bonded interactions

In this work we use Density-Functional Tight-Binding (DFTB) to investigate dynamical processes dependent on the flexibility in metal-organic frameworks (MOFs). We show that one can perform DFTB simulations on systems having unit cells of several hundreds atoms at a modest computational cost. This approach is validated by calculating the barriers to diffusion for small molecules (hydrogen, carbon dioxide, and methane) crossing the windows connecting the pores of ZIF-7 and ZIF-8, two prototypical materials that have been the subject of many experimental and theoretical investigations. Additionally, we calculate the vibrational densities of states for MOF-5 and ZIF-8. These calculations are compared with simulations using the bonded and non-bonded part of the Universal Force Field (UFF). The results show that UFF's description of the bonded interactions has a quality comparable to DFTB's, at an even smaller computational cost. (c) 2012 Elsevier Inc. All rights reserved.