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

Abstract

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, because of the complicate parameterization. Combining rules are usually suggested to reduce the parameterization by calculating the unlike-pair parameters from the information of like-pair. A new set of combination rules (DRS) for Morse-based force field has been proposed in our prior work and shown good performance in the simulations of CH4 adsorption isotherms in covalent-organic frameworks (COFs). Inspired by our prior work, we developed an accurate van der Waals force field by using high-level ab initio calculations with the DRS combination rules. The validation was conducted by comparing the simulated gas uptakes with experimental values for various known porous materials. The agreements between simulations and experiments are great, showing the potential application of the force field and the DRS combination rules.