Hello. I'm trying to calculate the total collisional cross section of the hydrogen atom impacted with the proton. (ion of the hydrogen plasma) I've found many good papers but they're showing specific cross section, for example, total cross section when n = 1 state is direct-excited to n = 2 when the proton impacts on the hydrogen atom. The original motivation of this calculation is to compute the diffusion coefficient of the ion in hydrogen plasma and I became confused of concept of the total cross section. In order to obtain proper cross section for my purpose, Should I sum up all cross sections for possible excitations or average them? I guess the latter is right since there are infinite possible excitations such that the all sum up should results in infinite value.