My understanding of the exchange correlation is basic. I know that the XC is the potential energy that keeps electrons in the same quantum coordinates {n,l,m and k (for periodic solids)} from having the same spin. Is there a analytical expression for the XC for say a 2-electron system such as He? In DFT calculations we approximate it by using the Local-Density-Approximation or Generalized-Gradient-Approximation. However, these expressions aren't correct and represent a major barrier in the calculation of the electronic ground-state. Thanks for any info.

Regards

Modey3