[grey_whiskers]

It's not a perturbation from that perspective, and a perturbation approach wouldn't be done that way: the perturbation would treat the atom as a hydrogenic atom with Z=22, and one electron. The second electron is then added as a perturbation. Just one additional electron is still a very large effect.

I thought perturbation theory for electronic structure was supplanted by configuration interaction; and that people generally liked to use Density Functional Theory rather than actually solving for the wavefunction explicitly.

Or am I just having a senior moment?

Cheers!

[Cyber Liberty]

Just realign the Flux Capacitor and you’ll be fine....

[FredZarguna]

No, you are correct. DFT is preferred these days.

I was taking his observation at face value: if you regard this as a perturbation how would you approach it.

However, for hydrogenic state vectors DFT is not needed; as you know, there's an exact solution, and for helium a perturbation is OK. I haven't taught it for a long time, but IIRC, the trick is usually tuning a Z_eff for each principal quantum number (n) then the two particle wave function "isn't too bad" even for angular momentum quantum numbers that have significant penetration into the inner shells.

Of course, that starts to crap out very quickly -- like, at Lithium -- so there isn't much of even pedagogic value for multiple electron systems in perturbation theory.