Gradient correction scheme for bulk and defect positron states in   materials: New developments
  As local density approximation positron calculations systematically underestimate positron lifetimes when they are compared with their experimental counterparts, the generalized gradient approximation (GGA) for positrons was introduced in the 1990s, in analogy with the GGA for electrons. New developments in the GGA for positrons are summarized and presented here and it is also discussed how they affect and possibly improve calculated positron lifetimes. In particular, these new GGA approaches are based on the recent perturbed hypernetted-chain and quantum Monte Carlo results. 
J. Kuriplach and B. Barbiellini
cond-mat.mtrl-sci