Metal-organic frameworks (MOFs) are promising for applications in gas storage, chemical separations, and catalysis. Given the millions of possible MOFs, computational high throughput screening (HTS) has emerged as a tool to rapidly identify promising MOF designs for which to attempt synthesis and experimental testing. HTS has proven successful, for instance, to predict application-relevant adsorption properties. However, a significant obstacle that remains when progressing from the identification of a promising (in silico) MOF prototype to actually synthesizing it is the uncertainty whether the prototype is synthesizable, due to an insufficient understanding of what makes a MOF thermodynamically and kinetically accessible.<br/>While the holistic view provided by HTS of adsorption properties has provided valuable insights on what determines the adsorption performance of a MOF prototype, similar insights on what makes a MOF prototype “synthesizable” are missing due to the absence of large-scale efforts to calculate measures of thermodynamic stability, such as free energy, in MOF databases. This gap is due to the relative complexity of free energy calculations in porous materials. Here we present our efforts to automatize free energy calculations of MOF prototypes using the Frenkel-Ladd (FL) path method. We compare free energies calculated using the FL method to those calculated using the harmonic approximation (HA), which is a method often used to calculate crystal free energies due to the simplicity of its principles. However, the HA is a multistep process that includes the calculation of the phonon density of states (PDOS), making it a relatively involved approach not ideal for large-scale screening.<br/><br/>Based on our simulation automatization efforts, we find the FL method to be better suited for calculation of free-energies in large MOF databases. Furthermore, the FL method does capture the contribution of anharmonic vibrations to crystal free energy, which are important in some structures. Indeed, we show that the entropic contributions to free energy calculated using the HA method tend to match poorly with those calculated using the FL method, which could result in substantial inaccuracies when considering highly flexible MOFs. Finally, we compare the free energies of a set of MOFs calculated using three “off-the-shelf” force-fields (including the MOF-focused UFF4MOF) to gauge how much different force field parameterizations affect MOF stability predictions.<br/>

Large-scale free-energy calculations in metal-organic framework prototypes

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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