Effect of Flexibility in Molecular Simulations of Carbon Dioxide Adsorption and Diffusion in a Cuprous Triazolate Framework

T Baucom and S Budhathoki and JA Steckel, JOURNAL OF PHYSICAL CHEMISTRY C, 127, 17524-17531 (2023).

DOI: 10.1021/acs.jpcc.3c03012

Using fixed atom force fields to model gas adsorption in flexible metal- organic frameworks (MOFs) is known to pose difficulties in accurately reproducing and predicting experimental results. This paper studies the difference in accuracy between flexible and fixed atom force fields in reproducing CO2 adsorption measurements in MAF-2 (Cu(etz)8 (MAF-2, Hetz) 3,5-diethyl-1,2,4-triazole), an NbO-type triazolate scaffold with a bcu cavity system and attached ethyl groups. The flexible force field used to run the hybrid molecular dynamics, and grand canonical Monte Carlo calculations were generated using QuickFF software incorporating van der Waals parameters from the universal force field (UFF) and density derived electrostatic and chemical (DDEC) charges. The fixed atom force field used was composed of UFF van der Waals parameters together with DDEC charges. The calculations were run at 298 K and pressures of 0.1, 0.3, 0.5, 0.8, and 1 bar. It was observed that for this MOF the rigid force field overpredicted gas adsorption, whereas the flexible force field values closely matched experimental data. In the flexible structure, the freely moving ethyl groups of MAF-2 hindered adsorption, reducing the interaction energy between CO2 and the N atoms of the triazolate framework as well as reducing the size of the largest cavity diameter. The combination of these factors led to improved prediction of adsorption values with the flexible force field as compared to the rigid force field, demonstrating the need for modeling MOFs in a way more indicative of their behavior.

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