Ground-state energy of quasi-free positrons in non-polar fluids

We have calculated the background energy (V₀) for positrons in noble gases with an ab initio potential and the Wigner-Seitz (WS) ansatz. In contrast to the general pseudo-potential approach, we have used accurate ab initio potentials for the positron-atom interaction. The ansatz includes an assumed form of the potential, resulting from an average over fluid atoms, and we propose four different options for this. By comparing the different options to literature data for an effective electron number (Z_eff), we find that agreement can be obtained for light elements but fails for heavy elements. We suspect that the strong polarizability of the heavy elements makes the simple potential averaging, as assumed in the WS model, insufficient to fit the measurements without also making use of pseudo-potentials. We also raise our suspicion that the comparison of annihilation rates between ground-state calculations and experimental values is not appropriate. Furthermore, the congruence of V₀ to Z_eff values predicted by a contact potential approximation appears to be invalidated by our results.