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

Area of Science:

• Materials Science
• Computational Chemistry
• Chemical Engineering

Background:

• Metal-organic frameworks (MOFs) offer tunable porosity for gas storage and separation.
• Grand canonical Monte Carlo (GCMC) simulations are crucial for predicting gas uptake in MOFs.
• Discrepancies between GCMC predictions and experimental data limit MOF screening.

Purpose of the Study:

• To systematically evaluate the impact of force field inaccuracies on GCMC simulation accuracy for gas uptake in MOFs.
• To assess the performance of the Universal Force Field (UFF) against experimental data for various adsorbates and MOFs.
• To investigate methods for refining force fields to improve GCMC predictive power.

Main Methods:

• Comparative analysis of UFF-based GCMC simulations with experimental gas adsorption isotherms (379 isotherms, 142 MOFs).
• Evaluation of four representative adsorbates: H2, CO2, C2H4, and C2H6.
• Development of a scheme to fit force field parameters using DFT-calculated energies.

Main Results:

• UFF consistently overestimates gas uptake in GCMC simulations across diverse MOFs and adsorbates.
• Force field refinement, while improving interaction energies, amplified the overestimation of experimental gas uptake.
• The study isolated force field inaccuracies as a key contributor to GCMC prediction errors.

Conclusions:

• Force field inaccuracies are a significant source of error in GCMC simulations of gas uptake in MOFs.
• Improving GCMC accuracy requires addressing not only force fields but also atomic motion and structural defects.
• Further research should focus on holistic approaches to enhance MOF simulation reliability.