Because the density uniquely defines them, BCPs constitute suitable points of comparison between levels of theory and experiment. BCPs are furthermore useful for characterizing bonds in different ways. For example, the sign of the Laplacian (Eq. ( )) at a BCP is an indication of local depletion (if positive) or concentration (if negative) of the electron density relative to its surroundings. A negative sign of ∇ $ 𝜌 indicates a covalent bond, while a positive sign hints at an ionic (or closed-shell) type of interaction.