The collision of particles without internal structure is the simplest model for interacting molecules. For the bulk, the model can account for the deviations from ideal gas behavior all the way to the formation of clusters. For systems in thermal disequilibrium the model describes the relaxation back to equilibrium. What is missing from the model is chemistry, that is the internal atomic configuration of the molecules. We will not forget this key point but we need to develop a language for thinking about how reactions take place. In order to undergo a reaction, the two reactants need to get close to one another and it is this approach motion, unhindered by any environment, that is discussed in this chapter. The angular distribution of the particles as they exit from the collision serves as a probe for the forces that acted between them when they were close by. Knowledge of these interactions is also needed for the prediction of the properties of liquids and solids and for understanding the conformation of large molecules.

In this chapter we use a two-body, A + B, point of view. But we have to leave the familiar vibration and rotation of a bound diatomic AB and go to the unbound or continuum motion. It is the vibrational displacement that is unbound. The rotation of the bound diatomic remains a rotation of the A-B axis but we will have to recognize that during the collision the A-B center-to-center distance varies over a wide range.