A method to calculate the "collision" frequency of molecules in the diluted solution can be proposed from my previous calculation on the adsorption rate of diffusive molecules or probes, in the bulk solution to fixed interface or surface, targets. The difference is now that both the probe and the target are moving. The first task of this report is to calculate the relative diffusion constant of two diffusive molecules A and B in a solution with diffusion constants DA and DB when using one as the reference, i.e., following it so it is not moving in its frame of reference and finding the relative diffusion of the other molecule, such that we can treat the reference as a fixed target and the other one as the diffusive probe. This is a classical Gaussian convolution problem with a standard solution that yields the relative translational diffusion constant,