1. The wavefunction cannot qualitatively be described by a single Slater determinant, and is "multiconfigurational." A chemical example is when two states of different character -e.g., ionic and covalent configurations that can be non-orthogonal along much of a dissociation coordinate -are of competing significance (energetically nearly degenerate). Another example from physics of such an intermediate regime involves fluctuations in materials such as transition metal oxides close to a Mott metal-insulator transition or f-block compounds, 7 in which electrons "hesitate" between delocalized (wave-like) and localized (particle-like) character. 2. The two-body interaction is large relative to one-body terms in the Hamiltonian. This has its origin in the Hubbard model: