Ab Initio, Physically Motivated Force Fields for CO2 Adsorption in Zeolitic Imidazolate Frameworks

We present an entirely ab initio methodology, based on symmetry adapted perturbation theory (SAPT), for constructing force-fields to study CO2 adsorption in nanoporous zeolitic imidazolate frameworks (ZIFs). Our approach utilizes the SAPT energy decomposition to generate physically motivated force fields for the CO2-ZIF interaction, with explicit terms representing exchange, electrostatic, induction and dispersion interactions. Each of these terms is fit to the corresponding term in the SAPT energy decomposition, yielding a force field entirely free of empirical parameters. This approach was utilized to construct force fields describing the CO2 interaction with both ZIF-8 and ZIF-71. In conjunction with our existing CO2-CO2 force field, parametrized in a consistent manner, we validate our force fields using grand canonical Monte Carlo simulations and obtain good agreement with the corresponding experimental CO2 adsorption isotherms. Furthermore, the explicit correspondence between force field terms and fundamental interaction types (dispersion, electrostatics, and induction) allows for an analysis of the underlying physics controlling ZIP gas adsorption that is far more direct and well-defined than with the generic force fields that had been previously utilized to study these systems. As our force fields are free from empirical parameters, these results demonstrate the potential for computationally screening novel ZIFs for flue gas separation applications with near quantitative accuracy.