As alluded to above, the characteristic features of inorganic electrides are still under debate, particularly when the charge concentration in the interstitial sites is activated by compression (e.g. HPEs). Nonetheless, it is reasonable to expect that crystalline inorganic electrides would possess the three aforementioned features of the electron density since they belong to the same category of materials. Therefore, it was surprising that Ca 24 Al 28 O 68 , the poster child of inorganic electrides, did not possess a NNM in the Ca-Al-O cages, nor a high value of the ELF. The very diffuse electron density inside the cavities has even led some authors to conclude that Ca 24 Al 28 O 68 is not characterized by the typical electride signatures, suggesting that the electride label may be inappropriate. Therefore, it is important to define meaningful guidelines, which are subsequently followed, to guarantee the unambiguous characterization of HPEs from quantum mechanical calculations.