Quantum Monte Carlo Study of a Positron in an Electron Gas
  Quantum Monte Carlo calculations of the relaxation energy, pair-correlation function, and annihilating-pair momentum density are presented for a positron immersed in a homogeneous electron gas. We find smaller relaxation energies and contact pair-correlation functions in the important low-density regime than predicted by earlier studies. Our annihilating-pair momentum densities have almost zero weight above the Fermi momentum due to the cancellation of electron-electron and electron-positron correlation effects. 
N. D. Drummond, P. Lopez Rios, C. J. Pickard, and R. J. Needs
cond-mat.stat-mech