As a summary of this section, we conclude that we have evidenced, by using the bond and band pictures, that the transformation from pre-MMB scenario at RP to the MMB scenario in and elements at HP proceeds by gradually increasing the electronic density. This result agrees with the view of Lubchenko and coworkers , but disagrees with the sudden change of coordination and properties between materials with covalent bonds and MMBs previously suggested by Wuttig and coworkers upon change in composition between PCMs and non-PCMs of the IV-VI and V2-VI3 families . Unlike what was previously assumed, we have shown that the transformation process involves several intermediate stages until fully (or almost fully) symmetrical MMBs are formed. In particular, we have evidenced that there is a mechanism of MMB formation that comprises three stages (two stages) in group-15 elements. The mechanism seems to be governed by the kind of LEP involved in the secondary interaction. Therefore, we conclude that the LEP stereoactivity, related to the s-p mixing, is the primary factor that rules the MMB formation in group-15 and -16 elements and by extension in binary IV-VI and V2-VI3 chalcogenides and other related complex chalcogenides. This result agrees with previous works that reflect the importance of s-p mixing in these elemental families . We consider that this universal mechanism is expected to apply to most materials evolving from the pre-MMB scenario (with primary covalent ppσ-bond plus a secondary interaction with the participation of LEPs) to the MMB scenario either due to compression or to change of composition by heavier elements. In particular, we predict that either a two or three-stage process for MMB formation at HP is likely to occur in other group-15 (N, P) and -16 (O, S) elements, also in group-17 elements (Br, I), and in IV-VI and V2-VI3 compounds that are not PCMs at RP. A hint of these stages has been observed in GeSe and As2S3 at HP . A more detailed discussion of the different stages of MMB formation in IV-VI and V2-VI3 compounds that are not PCMs at RP will be published elsewhere.