Please take a look at this link: http://books.google.dk/books?id=bA9Lp2GH6OEC&pg=PA178&lpg=PA178&dq="there+is+an+attractive+potential+between"&source=bl&ots=0Fx13N7BA9&sig=rpJFS4eCDGh2hgl_dVzXqQUGmU0&hl=da&ei=uNaeStf6DIKE-Qbvk4TaCw&sa=X&oi=book_result&ct=result&resnum=1#v=onepage&q="there 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?