# QM and van der Waals forces

1. Sep 2, 2009

### Niles

1. The problem statement, all variables and given/known data
Hi all

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 = $\hbar \omega_0$
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?

Last edited: Sep 2, 2009