To the best of our knowledge, this method has not been proposed before in the literature. It is closely related to several other approaches, however. On one hand, the trick of using the correlation energy per electron instead of the total correlation energy can also be used to derive the averaged coupled pair functional (ACPF) from the configuration interaction approach. In this sense, xBW2 can be seen as a secondorder approximation to ACPF. On the other hand, a similar second-order expression was also derived by Zhang, Rinke and Scheffler from the Bethe-Goldstone equation (BGE2). FIG. . Correlation energies per atom for evenly spaced (d=3 Å) Helium chains of increasing length, using the cc-pVDZ basis. A slope larger than zero indicates a non-size-extensive method.