, evaluating to zero when the four determinants have equivalent contributions at ϕ = π 8 . In these rare situations where non-uniqueness of the 1-RDM may occur at a single geometry, the trace does not have physical content, and the (more generally) orbital-invariant square norm of the 2 ∆ is to be preferred. In light of our numerical results and our assessment of the satisfaction of the properties listed above for most chemically-relevant electronic structure problems, both the trace and square norm of the 2 ∆ are apt metrics that reflect the statistical two-electron interdependence that we are proposing here as a meaningful interpretation of the term "strong correlation".