Traditional quantum chemistry begins with the mean field model, where the orbital approximation leads to a single Slater determinant. Electron correlation effects are then introduced perturbationally, or through coupled-cluster theory, or modelled with a density or density-matrix functional. When correlation is strong, multiconfigurational reference methods offer a general flexible approach that avoids the qualitative failure of the Hartree-Fock reference, but they have the disadvantages of strong cost scaling with system size, and, in most cases, a lack of size extensivity. In this lecture I will explore the extent to which the useful domain of single-reference methods can be extended. The consideration will include alternative coupled-cluster formulations, new forms of perturbation theory, density matrix functionals, and machine-learnt correlation models.
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