Quantum-mechanical properties at bond-critical points (BCPs) and their chemical interpretation.

Property	Formula	Interpretation	Ref.
Electron density	ρ	Bond order/strength of interaction	33 and 34
Laplacian	∇2ρ	Covalent character of interaction	34
Electron localization function	ELF	Degree of electron localization	35 and 36
Localized orbital locator	t σ	Degree of electron localization	37
Reduced density gradient	RDG	Homogeneity of electron density & type of interactions	38
sign (λ2)ρ	sign (λ2)ρ	Strength & type of interactionsa	38
Gradient norm	|∇ρ|	Magnitude of the gradient	39
Bond ellipticity	ε	Cylindrical symmetry and π-character of interaction	34
ETA index	η	Type of interaction	40

^a λ ₂ denotes the second-largest eigenvalue of the electron density's Hessian matrix.