where R is the radius of the collision cross-section (unit m), Dr is the relative diffusion constant (unit m 2 /s), CA and CB are the number concentrations (unit #/m 3 ), [A] and [B] are molar concentrations of molecule A and B respectively (unit mole/L, M), NA is the Avogadro's number, and kMS is the collision rate constant (unit M -1 s -1 ).