Recently, the guanidinium-based ionic liquid N,N,N′,N′,N′′-pentamethyl-N′′-propylguanidinium tetrafluoroborate ([ppg][BF4]) was found to be able to effectively absorb CO2 (Amitesh, M. ChemSusChem 2009, 2, 628). In this work, molecular dynamics (MD) simulations and ab initio calculations were performed to investigate the microscopic structure, interactions, and properties of pure ionic liquid [ppg][BF4] and mixtures of [ppg][BF4] (1) and CO2 (2) at x2 = 0, 0.1, 0.3, 0.5, 0.6. C–H···F hydrogen bonds were found to exist between [ppg] cations and [BF4] anions by ab initio calculations. However, the H···F coordination numbers suggest that the H···F interaction decreases slightly with increasing CO2 concentration. CO2 molecules were found to be primarily distributed around the anions, as seen from the CO2–cation and CO2–anion radial distribution functions (RDFs) calculated from molecular dynamic simulations. The cation–anion and anion–anion structures were not perturbed even with addition of CO2 up to 0.6 mol fraction, whereas the cation–anion and anion–anion interactions decreased with increasing CO2 concentration. The volume expansion and diffusion of [ppg][BF4] increased with increasing CO2 concentration. In addition, according to the results of RDFs and spatial distribution functions, the distribution areas of CO2 molecules and [ppg] cations around [BF4] anions were found to be nearly complementary.