ABSTRACT
Newton's equations of motion form the basis of the classical theory of collision. They are second order nonlinear differential equations in time, and their solution represents the time evolution of the coordinates of all particles. The estimate of the separation rate of two close trajectories has also more practical consequences. There is no simple, exactly solvable model which is typical for the unstable systems, and therefore, one can only guess at their properties. It is believed that because of the exponential separation in time of two initially close trajectories, the final distribution of states is random. In other words, shortly after the long-lived state is formed the trajectory "forgets" its initial conditions, and therefore any final state allowed by the integrals of motion is a priori equally probable. The use of Newton's equations of motion for the description of atom and molecule collisions has some obvious and some less obvious limitations.
