A nice paper on static electron correlation

I just read a very nice paper by Hollett and Gill: “The two faces of static correlation” and wanted to write a short post about it.
The article concludes that there are two types of static electron correlation: type A is the correlation energy that can be recovered by UHF. A good example for type A is stretched H₂. For infinite separation UHF can recover all the static correlation energy by breaking the spin symmetry of the wave function. Type B correlation is more subtle. Hollett and Gill show that Be-like ions with nuclear charge Z > 4.14 do not display the typical UHF triplet instability that characterize many near-degenerate problems. Nevertheless, these ions still have strong correlation effect that arises from the near-degeneracy of the 2s and 2p orbitals. Hollett and Gill call this type of static correlation Type B.