  We review our recent progress in the determination of the high-density
correlation energy $\Ec$ in two-electron systems. Several two-electron systems
are considered, such as the well known helium-like ions (helium), and the
Hooke's law atom (hookium). We also present results regarding two electrons on
the surface of a sphere (spherium), and two electrons trapped in a spherical
box (ballium). We also show that, in the large-dimension limit, the
high-density correlation energy of two opposite-spin electrons interacting {\em
via} a Coulomb potential is given by $\Ec \sim -1/(8D^2)$ for any radial
external potential $V(r)$, where $D$ is the dimensionality of the space. This
result explains the similarity of $\Ec$ in the previous two-electron systems
for D=3.
