The results in Fig. -e for chalcogens can be understood in the following way: In stage 2, the p-type LEP of Se and Te at RP shows two ELF basins aligned along two of the four secondary bonds. This stage of chalcogens mirrors stage 2 of pnictogen, where the lobes are also properly aligned along the secondary bonds. Therefore, it seems that group-15 and -16 elements show a different number of stages that are related to the different LEP distribution (or kind of LEP) involved in the secondary bonds. Moreover, the relation between stage 2 in both pnictogens and chalcogens is now clear and it can be understood why the short primary bond distance (charge density) shows an anomalous increase (decrease) with increasing pressure in both cases at this stage. The reason is the strong trans influence present in this stage of the pre-MMB to MMB transformation that is also responsible for the strong decrease of the A1 phonon frequencies in Se and Te already at RP. All the above results are consistent with previous results for Te at HP . Finally, the trans influence ends at Pt when the secondary bonds acquire equal or almost equal charge as the primary bonds (onset of stage 3). As previously mentioned for pnictogens, the p-type LEP of chalcogens in stage 2 becomes gradually inactive as pressure increases due to the LEP depopulation/delocalization; however, the behavior of the LEP of chalcogens at this stage is different from that of pnictogens. In stage 2 of chalcogens, the ELF basins of the p-type LEPs become increasingly distorted with increasing pressure and become elongated towards the direction of the two secondary bonds with no ELF attractor at RP. This behavior is consistent with the electron transfer of the p-type LEP to the antibonding orbitals in Te-I at HP reported in Ref. and also with the breakdown and delocalization of the LEP in Te-I at HP, so that these electrons become itinerant in Te-II as suggested in Ref. . These results contrast with s-type LEPs in group-15 elements that barely change their profiles in stage 2 as they become progressively inactive. Due to the itinerant electrons, the ELF isosurface of Se and Te at the end of stage 2 shows a toroidal shape around each atom that is maintained in stage 3 and is characteristic of the Ai phase and different from that of the Ah phase, where no itinerant electrons are observed. These itinerant electrons in the Ai phase can be ascribed to the extra valence electron present in group-16 elements as compared to group-15 elements. Therefore, it is clear why group-15 elements with five valence electrons tend to the Ah phase at HP, as rationalized by Papoian and Hoffmann , and why group-16 elements with six valence electrons tend to the Ai phase at HP. Now we are in a better position to show the correlation between the three (two) stages found in group-15 elements with the three stages represented in Fig. and and their extension to IV-VI and V2-VI3 compounds related to PCMs. Stage 1 in Fig. corresponds to a material with primary, short, and strong covalent 2c-2e ppσ-bonds (distance d1) between A and B atoms and secondary, large, and weak bonds to atom C (distance d2), which usually corresponds to a ratio d2/d1≫ This stage occurs in materials showing single-basin-type LEPs, such as pnictogens at RP, and it could be also the case for isoelectronic IV-VI compounds showing no rs structure at RP (GeS, GeSe, α-GeTe, SnS, and SnSe) as well as V2-VI3 compounds showing no tetradymite-like structure at RP (As2S3, As2Se3, α-As2Te3, Sb2S3, Sb2Se3, and Bi2S3). The step going from stage 1 to stage 2 is characterized by the single-basin-type LEP dissociation or splitting, i.e. the LEP in stage 1 splits into several basins aligned along the directions of the secondary bonds. Therefore, stage 2 (Fig. ) corresponds to a material with weakened primary covalent ppσ-bonds and strengthened secondary bonds caused by the charge transfer (trans influence) from the primary bonds to the secondary bonds. This stage occurs upon the decrease of the d2/d1 ratio (d2/d1 > 1) caused by either the effect of pressure or composition. Stage 2 is the step before the formation of the MMB and asymmetric MMBs are likely formed in stage 2 for values of d2/d1 close to 1 (tentatively below 1.05 -1.10). Stage 2 is observed in materials with p-type LEPs at RP, such as Se and Te, and also in materials with single-basin-type LEPs at HP, such as pnictogens and perhaps the above commented IV-VI and V2-VI3 materials. The step going from stage 2 to stage 3 is characterized by the disappearance (or almost) of the LEP stereoactivity, which finally leads to octahedral coordination in materials with p-type bonds. Curiously, the decrease of the LEP stereoactivity in group-15 and -16 elements at HP on approaching the octahedral coordination is related to the appearance of superconductivity in all these elements at HP, except for Bi, which already shows superconductivity at RP . Finally, stage 3 (Fig. ) ideally to a material with fully developed and symmetric MMBs and with equal A-B and B-C bonds that have one of the hypercoordinated multicenter geometries represented in Fig. . This stage occurs for d2/d1≈ 1. For instance, stage 3 occurs in the Ah and Ai phases of Po at RP and in octahedrally-coordinate pnictogens and chalcogens at HP. This stage with MMBs is also observed at RP in the rs phase of IV-VI PCMs and of related ternary compounds, such as GeSb2Te4 and GST. Moreover, stage 3 can be considered to be also present at RP in the tetradymite-like phases of V2-VI3 PCMs (Bi2Se3, β-As2Te3, Sb2Te3, and Bi2Te3) and of ternary IV-V2-VI4 compounds (GeSb2Te4, SnSb2Te4) , although not all atoms in the tetradymite-like phase show MMBs due to the layered nature of the tetradymite-like phases. The above observations support the idea already mentioned that the LEP stereoactivity decreases at HP and on going down a group in the periodic table. A total disappearance of the LEP stereoactivity is noticed in stage 3 if a cubic phase is obtained, such as in the Ah structure (of P, As, and α-Po) or the rs structure (of IV-VI PCMs). However, a residual LEP stereoactivity is observed in stage 3 if a distorted cubic phase is obtained, such as the distorted sc phases (of Bi-II and Bi-III) and the rhombohedral phases (of group-16 elements and tetradymite-like V2-VI3 PCMs).