On the other hand, Wuttig and coworkers have proposed that PCMs feature a new unconventional bonding characterized by a mixture of localized electrons (as in covalent materials) and delocalized electrons (as in metals); i.e. a bond that is intermediate between the 2c-2e bond and metallic bonds and is argued to be a kind of two-center-one-electron (2c-1e) bond . Consequently, these authors coined the term "metavalent" bond and considered the materials with this bond as incipient metals. Interestingly, they located incipient metals in an intermediate position between covalent and metallic materials in a 2D map showing the number of electrons shared (ES) vs. the normalized number of electrons transferred (ET), which are defined using Bader's integrated atomic charges and delocalization indices from the Quantum Theory of Atoms in Molecules (QTAIM) .