when the 1 Γ and 2 Γ are traced to N and N (N -1), respectively, and may be calculated explicitly as the non-terminating block components of the full 2-RDM or from the spintraced, sometimes called charge, density matrix. The cumulants of the density have several properties that make them useful in the construction of a metric for strong correlation. For example, NOONs have been proposed as a way to identify active space orbitals, since partial occupation of orbitals is considered an identifier of multiconfigurational character. This strategy first appeared with regard to the broken spin-symmetry mean-field level of calculation. However, as first noted by Juhász and Mazziotti, 53,58 the 1 ∆ contains insufficient data to describe the entire spectrum of possible strong correlation effects and offers no unique signature for infinitely separated hydrogen atoms with ⟨S 2 ⟩ = 0. Interestingly, ∆ is able to yield the diagonal elements of 2 ∆, and hence its trace and statistical dependence of pairwise orbitals, although problems were encountered for mixed/ensemble states. Alternatively, the squared Frobenius norm of the 2 ∆ was proposed by Mazziotti as a size-extensive quantity invariant to unitary transformation of the orbitals which can capture the spin-entanglements of atom centers at separation, while the trace and eigenvalues of 2 ∆ may capture the rise of ODLRO. Further studies quantifying strong correlation utilizing the 2 ∆ have been proposed, including metrics based on a two-electron two-fragment model in a minimal basis and other terms derivable from the 2 ∆ such as the cross-trace and infinity-norm. 62,63