1. The problem statement, all variables and given/known data Hi all Please take a look at this link: http://books.google.dk/books?id=bA9...e is an attractive potential between"&f=false I have a question for the above-mentioned case with van der Waals interaction: In my QM-book (Griffiths, problem 6.31), we look at a system of two neutral atoms a distance R apart. Treating the Coulomb-interaction as a perturbation, we have the following values: Eunperturbed = [itex]\hbar \omega_0[/itex] Eperturbation, total = Eunperturbed + Efirst order Thus we have Eperturbed, total-Eunperturbed = Efirst order < 0. Now the author concludes that this implies an attractive potential between the atoms, and this is called the van der Waals interaction. I don't understand why we are able to conclude that?