hi everyone! I am so confused about interaction energy! as you know, in many statistical mechanics books we see there are equations which are taken in a non-interacting system. for example for a two particle system, you can see the total energy is: E = E1 +E2 +E12 while E12 is the interaction energy between particle 1 and particle 2. What I wanna know is that when can we neglect this interaction energy? how far is interaction energy's range? can anybody give me a formula or something for it?!!