Making some simple assumptions about spherical collisions leads to the bi-molecular collision rate:

(3–21)

Where is Avogadro’s constant, is the average diameter of the two spherical particles, is the universal gas constant, is temperature, is the reduced molar mass of the two species, and is the concentration of the species .

This expression is the theoretical limit that two spherical entities A and B collide. Using this as a basis for chemical kinetic rates, a formulation for a reaction rate constant is derived.

(3–22)

The new term is a correction factor, sometimes called the probability factor or the steric factor. This factor encompasses the probability of a bimolecular collision leading to reaction, and for Ansys Chemkin the factor is expressed in Arrhenius form.

(3–23)

This expression limits the probability factor to be less than or equal to one. A probability factor of unity means that every collision leads to reaction, which is valid for many radical-radical recombination reactions, but represents an upper limit for other bimolecular reactions.