Collisional energy transfer of highly vibrationally excited propylbenzene cation in a N 2 bath has been studied with chemical dynamics simulations. In this work, an intermolecular potential of propylbenzene cation interacting with N 2 was developed from SCS-MP2/6-311++G∗∗ ab initio calculations. Using a particle swarm optimization algorithm, the ab initio results were simultaneously fit to a sum of three two-body potentials, consisting of C a -N, C b -N, and H-N, where C a is carbon on the benzene ring and C b is carbon on the propyl side chain. Using the developed intermolecular potential, classical trajectory calculations were performed with a 100.1 kcal/mol excitation energy at 473 K to compare with experiment. Varying the density of the N 2 bath, the single collision limit of propylbenzene cation with the N 2 bath was obtained at a density of 20 kg/m 3 (28 atm). For the experimental excitation energy and in the single collision limit, the average energy transferred per collision, E c , is 1.04 ± 0.04 kcal/mol and in good agreement with the experimental value of 0.82 kcal/mol.