Development and Implementation of an Enhanced Free Energy Perturbation Approach to Rapidly Assess Gas Solubility in Charged Liquids

Gas solubility simulations are useful for guiding the discovery of useful materials and conditions for processes involving gas absorption or adsorption. We present an efficient, general-purpose variant of the test particle insertion method for computing chemical potentials of gaseous solutes in fluids or porous solids. The method is implemented in the Monte Carlo molecular simulation engine Cassandra, but receiving phase configurations are independent of this process and may be pre-sampled by other molecular simulation engines such as molecular dynamics codes. Efficiency enhancement options present in this method include configurational biasing, various accelerated atomic overlap detection methods, and bitmapped cavity biasing. The method's implementation is extremely optimized compared to prior configurational biasing implementations in Cassandra. When applied to the estimation of Henry's law constants of atomistic hydrofluorocarbons in ionic liquids, the speed enhancements and implementation optimizations can result in a speedup of more than three orders of magnitude compared to conventional configurational biasing methods without sacrificing accuracy. We found good agreement between this method and Hamiltonian replica exchange (HREX) for Henry's law constant and absorption isotherm estimation. This embarrassingly parallel method is especially well suited for screening Henry's law constants of many small gases in the same solvents, since a liquid trajectory can be reused for as many solutes as desired. We apply this method to fast, high-throughput calculation of Henry's law constant for 16 small gases in 23 ionic liquids. We recommend future use of this method for additional studies of gas solubility in liquids and porous solids due to its extreme efficiency.