From the above discussion, it is clear that electrides must possess some electron density localized in interstitial pockets, and not along the contacts between atoms that could correspond to covalent bonds, nor within regions of space associated to lone electron pairs. Therefore, it is sensible to characterize the electride state based upon the electron density, ρ(r). It was shown that molecular electrides must possess three specific topological features in the electron density, defined and quantified by the Quantum Theory of Atoms in Molecules (QTAIM), and by the Electron Localization Function (ELF). These topological features are: