The direct simulation Monte Carlo (DSMC) method has, in recent years, become widely used for engineering and scientific studies of gas flows that involve low densities or very small physical dimensions. The method is a direct physical simulation of the motion of representative molecules, rather than a numerical solution of the equations that provide a mathematical model of the flow. The computations are no longer expensive and the period since the original 1976 publication of this work has seen enormous improvements in the molecular models, the procedures, and the implementation strategies....