Combination Rules and Accurate van der Waals Force Field for Gas Uptakes in Porous Materials.

Morse function is suggested to be more suitable for studying the gas adsorption in porous frameworks than the Lennard-Jones and exponential-6 forms. However, there have not been some widely used Morse-based van der Waals force fields (vdW FFs) because of complicated parameterization. Combining rules is usually suggested to reduce the parameterization by calculating the unlike-pair parameters from the information of the like pair. A new set of combination rules (DRS) for Morse-based FF has been proposed in our prior work and shown good performance in the simulation of CH4 adsorption isotherms in covalent organic frameworks. Inspired by our prior work, we developed an accurate van der Waals FF using high-level ab initio calculations with the DRS combination rules. The validation was conducted by comparing the simulated gas uptakes with the experimental values for various known porous materials. The agreement between simulations and experiments is very good, showing the potential application of the FF and the DRS combination rules.