Molecular dynamics modeling of hypersonic gas-phase and gas-surface reactions

Efforts to use molecular dynamics (MD) to develop both non-equilibrium dissociation models required in the shock layer as well as gas-surface interaction models specifically for surface catalysis will be summarized. First, an accelerated MD algorithm for dilute gases is presented, called the Event-Driven/Time-Driven (ED/TD) MD method. The method detects and moves molecules directly to their impending collision while still integrating each collision, including multi-body collisions, using conventional Time-Driven (TD) MD with an arbitrary inter-atomic potential. The simulation thus proceeds at time steps approaching the mean-collision-time. Preliminary nonequilibrium relaxation and normal shock wave simulations are in excellent agreement with direct simulation Monte Carlo (DSMC) results with large speedups over conventional TD MD, especially at low densities. Second, an MD simulation technique to study surface catalysis employing the ReaxFF inter-atomic potential is detailed. SiO₂ surfaces are equilibrated with a dissociated gas mixture at various temperatures and pressures, establishing surface coverage. Rates of dominant reaction mechanisms, including adsorption, desorption, and E-R/L-H recombination, are then determined by counting individual events. The experimentally measured exponential dependence of recombination coefficient on temperature is well predicted by the MD simulations.