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## Homework Statement

The van der waals forces describe the forces between neutral atoms, separated by a distance much larger than the typical size of a single atom, as we saw in class.

Therefore we describe the interaction between the atoms by: [tex]W=W_{dd}+W_{dq}+W_{qd}+W_{qq}+...[/tex] i.e W is the energy sum which has contributions from dipole-dipole, quadrapule-dipole, quadrapule-quadrapule and so.

In class we were shown that the first order correction due to dipole dipole interaction [tex]W_{dd}[/tex] vanishes, and we solved this with second order perturbation theory.

But this is actually justified if first order correction of all other contriubtions to W vanish as well. Show that this is the case here.

## Homework Equations

## The Attempt at a Solution

I know that I need to find the quadrapule tensor Q for two atoms, but not sure how does this look like? i.e I know the general equation for the potential of a quadrapule and the euqation for the quadrapule itself, but don't know how to write Q explictly.

P.s

I am using Cohen-Tanoudji notation and his coverage of Vand Der Waals forces, in Vol 2, page 1130 complement Cx1.

Thanks in advance.