This method has some formal advantages over MP2. It yields the exact correlation energy for two-level systems and avoids the divergence of the energy for vanishing ∆ ab i j . There was therefore considerable interest in the Brillouin-Wigner series in the early days of quantum chemistry, which persists in the context of multi-reference perturbation theory. However, BW2 also has a considerable downside, namely that it is not size-extensive. This is obviously problematic for applications to extended systems. Nonetheless, the idea of using a correlation energy dependent regularization is attractive, since diverging correlation energies (e.g. beyond Coulson-Fisher points in bond-breaking) are a clear signal that regularization is needed.