The rate constant is then calculated as ⁠ k = z A B e − − E a R T ⁠, so that the collision theory predicts that the pre-exponential factor is equal to the collision number z AB. However for many reactions this agrees poorly with experiment, so the rate constant is written instead as ⁠ k = ρ ρ z A B e − − E a R T ⁠. Here ρ ρ 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.