The direct simulation Monte Carlo algorithm is like molecular dynamics in that the state of the system is given by the positions and velocities of the particles, { r i , v i } , for i = 1 , … … , N . Unlike molecular dynamics, each particle in a DSMC simulation represents F N molecules in the physical system that have roughly the same position and velocity. This allows DSMC to rescale length and time for the modeling of macroscopic systems (e.g., atmospheric entry). Specifically, the system volume is V = ( N F N ) / n , where n is the number density and each collision between simulation particles represents F N collisions among molecules in the physical system. As a rule of thumb there should be 20 or more particles per cubic mean free path for accurate results.