  We have calculated the background energy ($V_0$) for positrons in noble gases
with an \emph{ab initio} potential and the Wigner-Seitz (WS) ansatz. {\col In
contrast to the general pseudo-potential approach, we have used accurate
\emph{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 effective electron number
($Z_{\text{eff}}$), 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
Wigner-Seitz model, insufficient to fit the measurements without also making
use of pseudo-potentials. We also raise our suspicion that the comparison of
annihiliation rates between ground-state calculations and experimental values
is not appropriate. Furthermore, the congruence of $V_0$ to $Z_{\text{eff}}$
values predicted by a contact potential approximation appears to be invalidated
by our results.
