The Relationship Between Entanglement, Energy, and Level Degeneracy in   Two-Electrons Systems
  The entanglement properties of two-electron atomic systems have been the subject of considerable research activity in recent years. These studies are still somewhat fragmentary, focusing on numerical computations on particular states of systems such as Helium, or on analytical studies of model-systems such as the Moshinsky atom. Some general trends are beginning to emerge from these studies: the amount of entanglement tends to increase with energy and, in the case of excited states, entanglement does not necessarily tend to zero in the limit of vanishing interaction between the two constituting particles. A physical explanation of these properties, shared by the different two-electrons models investigated so far, is still lacking. As a first step towards this goal we perform here, via a perturbative approach, an analysis of entanglement in two-electrons models that sheds new light on the physical origin of the aforementioned features and on their universal character. 
A. P. Majtey, A. R. Plastino, J. S. Dehesa
quant-ph