A chemical dynamics simulation was performed to study low energy collisions between N 2 and a graphite surface. The simulations were performed as a function of collision energy (6.34 and 14.41 kcal/mol), incident polar angle (20−70°) and random azimuthal angle. The following properties were determined and analyzed for the N 2 + graphite collisions:(1) translational and rotational energy distributions of the scattered N 2 ; (2) distribution of the final polar angle for the scattered N 2 ; (3) number of bounces of N 2 on the surface before scattering. Direct scattering with only a single bounce is dominant for all incident angles. Scattering with multiple collisions with the surface becomes important for incident angles far from the surface normal. For trajectories that desorb, the parallel component of the N 2 incident energy is conserved due to the extremely short residence times of N 2 on the surface. For scattering with an incident energy of 6.34 kcal/mol, incident polar angle of 40°, and final polar angle of 50°the percentage incident energy loss is 29% from the simulations, while the value is 27% for a hard cube model used to interpret experiment (J. Phys.: Condes. Matter 2012, 24, 354001). The incident energy is primarily transferred to surface vibrational modes, with a very small fraction transferred to N 2 rotation. An angular dependence is observed for the energy transfer, with energy transfer more efficient for incident angles close to surface normal.

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In this work a particle swarm optimization (PSO) algorithm was used to fit an analytic potential energy function to I(HO) intermolecular potential energy curves calculated with DFT/B97-1 theory. The analytic function is a sum of two-body terms, each written as a generalized sum of Buckingham and Lennard-Jones terms with only six parameters. Two models were used to describe the two-body terms between I and HO: a three-site model HO and a four-site model including a ghost atom. The fits are compared with those obtained with a genetic/nonlinear least-squares algorithm. The ghost atom model significantly improves the fitting accuracy for both algorithms. The PSO fits are significantly more accurate and much less time-consuming than those obtained with the genetic/nonlinear least-squares algorithm. Eight I---HO potential energy curves, fit with the PSO algorithm for the three- and four-site models, have RMSE of 1.37 and 0.22 kcal/mol and compute times of ∼20 and ∼68 min, respectively. The PSO fit for the four-site model is quite adequate for determining densities of states and partition functions for I(HO) clusters at high energies and temperatures, respectively. The PSO algorithm was also applied to the eight potential energy curves, with the four-site model, for a short time ∼8 min fitting. The RMSE was small, only 0.37 kcal/mol, showing the high efficiency of the PSO algorithm with retention of a good fitting accuracy. The PSO algorithm is a good choice for fitting analytic potential energy functions, and for the work presented here was able to find an adequate fit to an I(HO) analytic intermolecular potential with a small number of parameters.

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, ⟨ΔE c ⟩, is 1.04 ± 0.04 kcal/mol and in good agreement with the experimental value of 0.82 kcal/mol.

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