A great deal of attention has been recently devoted to the analysis of the chemical bonding in phase change materials (PCMs) consisting of binary chalcogenides of the IV-VI and V2-VI3 families, such as GeTe, SnTe, Sb2Te3, and their related ternary compounds, such as Ge2Sb2Te5 (GST). PCMs have an exceptional property portfolio that includes: i) hypercoordination (violation of the 8-N rule); i.e. a higher atomic coordination than expected for compounds with covalent ppσ-bonds that do obey the 8 -N rule, ii) relatively low band gaps and shiny metallic luster, iii) moderate electrical conductivity, iv) extremely high optical dielectric constants and Born effective charges, v) low-frequency optical phonons and high Grüneisen parameters, vi) low thermal conductivity, and vii) brittleness and high probability of multiple emission events in laser-assisted field evaporation measurements, which has been attributed to the softer character of their bonds than of covalent ones . Thanks to these unconventional properties, PCMs have a wide range of applications such as re-writable data storage in DVDs, phase change RAM memories, and thermal energy storage systems . Moreover, many PCMs also show highly efficient thermoelectric and exceptional topological properties . Until the last decade, the unconventional chemical bonding in PCMs was considered to be a kind of resonant bonding related to the single covalent (two-center-two-electron, 2c-2e) bond . This was similar to that suggested by Pauling for benzene and graphite (latter extended to metals) with the possible participation of d orbitals. However, from the 1980 decade on, it was noted that: i) the contribution of d orbitals to the unconventional bonding in PCMs is minor , ii) the influence of s-p mixing, related to the LEP stereoactivity, in the bonding of PCMs is essential , and iii) the properties of PCMs are completely different from those of benzene and graphite .