Hard Spere (HS) Model
This model is best visualized as a collection of hard, perfectly elastic, balls. There is no force acting on the particles except at the instant when they are some distance (s) apart. Thus HS model can be viewed as an approximaiton to the LJ model with high-velocity particles. HS particles travel in straingt lines with constant speed between collisions. In order to follow the motions of the particles(atoms), we only need to know when collisions occur and the velocities after collision. For a more complete explanation of the Hard Sphere model read "Molecular Dynamics: An Introduction" by Lloyd Fosdick.