A possible solution, collision theory, has been developed independently by Max Trautz in 1916 and William Lewis in 1918. It has been very successful in explaining the reaction kinetics in the gas phase when there is no dilution. The basic idea is for two molecules, one can calculate their collision frequency using models such as a simple hard sphere collision model, and then calculate the reaction probabilities of all possible collision geometries. A thought collision experiment of gas molecules A and B under normal conditions is shown in Fig. . The collision frequency is calculated from the randomly oriented speeds of the molecules, v, that are Maxwell-Boltzmann distributed, the density of the molecules, and the cross-section of the collisions. It is like the probability of randomly shooting a ball to a matrix of balls. The average distance of the free of the fly before hitting a ball is called the mean free path. Typically, you would expect to hit a ball within nanoseconds of fly under normal conditions, with a mean free path of ~80 nanometers and an average speed of ~500 meters per second for a small molecule like oxygen. The collision frequency is dependent on the product of the concentrations of the two molecules, as many scientists including Richard Feynman have explained in their lectures. The probability of the reaction of each collision can be calculated using the transition state theory either classically or quantum mechanically. It focuses on the collision geometry near the dashed line in Fig. , when two molecules approach each other and calculate the activation energy and collision geometry needed to break old chemical bonds and form new bonds. However, there is still a gap in knowledge of collision theory nowadays. When an inert molecule C is introduced as a solvent to dilute the molecules and form a solution, the time scale for the collision between the molecules of interest and the distance time trajectory (Fig. ) looks very different than the scenario Fig. illustrates. There are no clear trajectories for the collisions and the simple argument on time of fly fails. The frequency of finding the other molecules now is controlled by diffusion and it becomes one of the dominating steps for the reaction kinetics. For example, if we consider the trajectory of A finding B in a solution of inert molecules C, it often collides with many C molecules, changing speed and directions many times before finding a B, i.e., a Brownian motion of diffusion. This diffusion process has been historically described by many models such as the "drunken sailor" random walk model proposed by scientists including Albert Einstein. Nowadays, this process is usually studied using molecular dynamic simulations. However, there is a technical difficulty for the dynamic simulation to predict the reaction kinetics. When the molecule is too diluted, for example, at the millimolar level in the water, it will take milliseconds for a collision of interest to happen which is challenging the computational power of our current computers.