8. The energies from Moller-Plesset perturbation theory through low orders with the generalized Hartree-Fock (GHF) reference state qualitatively differ from that of Full Configuration Interaction (FCI). The causal relationship between strong correlation and the breakdown of many-body perturbation theory is relatively well recognized in the physics literature. The failure of approximate density functional theory (DFT) is another common characterization. Alternatively, a colleague recently exclaimed of strong correlation that "you know it when you see it"! While Small and Head-Gordon have defined strong correlation in terms of the breakdown of a specific perturbation theory, there is general agreement that a consequence of strong correlation is that feasibly-scaling electronic structure models such as low orders of single-reference perturbation or coupled cluster theories and approximate density functionals (especially those from the higher rungs of Jacob's ladder) break down. Thus it is of practical value to computational scientists to reliably diagnose when a system shows symptoms of strong correlation. For example, a recent work has delineated correlation regimes inside of which CCSD(T) is expected to produce ionization potentials of transition metal complexes to sub-kcal/mol accuracy, and outside of which CCSD(T) is expected to qualitatively fail (no matter which orbitals are used in the reference determinant). Such predictive classification relied on a composite diagnostic involving both spin-symmetry breaking in unrestricted CCSD wavefunctions and the largest CI coefficient in selected CI wavefunctions (in the basis of natural orbitals).