Therefore, we first want to give a short overview on the importance of topological analyses for the recognition of electrides, which is discussed in more details in prior reviews . To date, there is not a general consensus for a unique definition of HPEs, and many theories have been developed to classify and predict this class of materials in recent years . Nonetheless, it is established that electrides must possess some specific topological features : 1) the presence of a non-nuclear maximum (NNM) in ρ(r); 2) a largely negative Laplacian of the electron density,  2 ρ(r), at the NNM critical point; and 3) it must be possible to associate a NNM to an ELF basin having a value as close as possible to 1. Ca-sc satisfies all the topological requirements to be classified as an HPE.