When dealing with electronic correlations in solids, one finds that they often resemble those in corresponding molecules or clusters. Hence one would expect quantum chemistry and solid-state theory to be two areas of research with many links and cross fertilization. Regrettably this is not the case. The two fields have diverged to such an extent that it is frequently difficult to find even a common language, something we hope will change in the future. In particular it has become clear that the various methods applied in chemistry and in solid-state theory are simply different approximations to the same set of cumulant equations.Peter Fulde, Correlated electrons in quantum matter (World Scientific, 2012), pages 3-4.