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 a. The number of binary collisions between two unlike molecules per second per unit volume is found to be z A B = N A d A B 2 8 π π k B T μ μ A B , where N A is the Avogadro constant, d AB is the average diameter of A and B, T is the temperature which is multiplied by the Boltzmann constant k B to convert to energy, and μ AB is the reduced mass.