One approach is the collision theory of chemical reactions, developed by Max Trautz and William Lewis in the years 1916–18. In this theory, molecules are supposed to react if they collide with a relative kinetic energy along their line of centers that exceeds E. The number of binary collisions between two unlike molecules per second per unit volume is found to be
where N is the Avogadro constant, d is the average diameter of A and B, T is the temperature which is multiplied by the Boltzmann constant k to convert to energy, and μ is the reduced mass.
The rate constant is then calculated as , so that the collision theory predicts that the pre-exponential factor is equal to the collision number z. However for many reactions this agrees poorly with experiment, so the rate constant is written instead as . Here  is an empirical steric factor, often much less than 1.00, which is interpreted as the fraction of sufficiently energetic collisions in which the two molecules have the correct mutual orientation to react.