A new theoretical method was developed to compute the Henry's law constant for gas absorption in a solvent through strong nonphysical interactions. The new method was created by expanding the test particle insertion method typically applied to physisorbing systems to account for the strong intermolecular interactions present in chemisorbing systems. By using an ab initio (AI)-based Boltzmann-averaged potential to model the interaction between CO2 and the tetra-n-butylphosphonium acetate ([P-4444] [CH3COO]) ionic liquid, the total Henrys's law constant at 298 K was computed to be 0.011 to 0.039 bar, reasonably comparable to the experimental value of 0.18 bar measured in this work. Three different All potentials were used to verify the applicability of this approach. In contrast, when a classical force field (FF) was used to describe the interaction between CO2 and [P-4444][CH3COO], the Henry's law constant was computed to be 27 bar, significantly larger than the experimental value. The classical FF underestimates the CO2 [P-4444][CH3COO] interaction compared with the AI calculations, which in turn leads to the smaller simulated CO2 solubility. Simulations further indicate that the CO2 interaction with the [CH3COO](-) anion is much stronger than with the [P-4444](+) cation. This result strongly suggests that the large CO2 solubility in [P-4444][CH3COO] is due to the strong CO2 [CH3COO](-) interaction.