Hi people, I'm researching about the interactions of two carbon atoms using the Lennard-Jones potential and I need to know the theory behind some equations. I need to know how to get from the 6-12 potential the 3-9 one. I've found in this link ( http://www.sklogwiki.org/SklogWiki/index.php/9-3_Lennard-Jones_potential ) a brilliant deduction of what I want applying triple integrals. I understand everything except the limits of the integral respect to z. From -x to -∞? Why's that? If someone could throw some light to this issue I would be really grateful. Thank you in advanced. Kind regards!