Assessing UFF and DFT-Tuned Force Fields for Predicting Experimental Isotherms of MOFs

Metal–organic
frameworks (MOFs) are promising
materials
for gas storage and separation applications due to their high tunability
and porosity. The rational design of MOFs relies on accurate computational
modeling, with grand canonical Monte Carlo (GCMC) simulations frequently
employed to model gas uptake. However, GCMC predictions often deviate
from experimental observations, limiting their utility in MOF screening.
These discrepancies primarily arise from three factors: inaccuracies
in the force field, neglect of atomic motions, and neglect of structural
imperfections in MOFs. In this study, we systematically evaluate the
impact of the first factor on the predictive accuracy of the GCMC
simulations. We evaluate the widely used Universal Force Field (UFF)
by comparing its predictions with experimental isotherms for four
representative adsorbates, H2, CO2, C2H4, and C2H6, across 379 isotherms
from 142 MOFs. The results show that UFF consistently overestimates
the gas uptake in GCMC simulations. To isolate the contribution of
force field inaccuracies to errors in GCMC, we developed a practical
scheme for fitting force field parameters to DFT-calculated energies
for a large set of MOFs. While the refined force field improves the
accuracy of interatomic interaction energies, its reduction of repulsion,
combined with UFF’s tendency to overestimate gas uptake, ultimately
amplifies the overestimation of experimental gas uptake meaurement.
Our analysis suggests that improving the agreement of gas adsorption
prediction with experiments requires addressing atomic motion and
structural defects in MOFs alongside force field refinements.