Group-16 elements. Experimental and theoretical data show that the pressure-induced A8-to-Ai PT in Se and Te is characterized by the equalization of the two short primary intrachain (d1) and the four large secondary interchain (d2) bond distances as pressure increases (Fig. and S10a in ESI). Concomitantly, an equalizing trend is observed for ρd1 and ρd2 (Fig. and) and for ELFd1 and ELFd2 at BCPs (Fig. -e in ESI). The most important point is that two stages occur in Se and Te at HP in the above-mentioned figures that correspond to stages 2 and 3 in Fig. . In stage 2 (near RP), there is a strong decrease of d2 and an anomalous increase of d1 that matches with available experimental values for Se (see Fig. ) . This anomalous increase of d1 is similar to that previously commented of As in stage 2 above 16 GPa. Correspondingly, there is an increase (decrease) of ρd2 (ρd1) in Se and Te, respectively. Finally, in stage 3, the Ai phase is reached and both primary and secondary bonds of Se and Te become MMBs that show a normal decrease (increase) of the bond distance (bond charge density) at HP. Again, the partial S-like behavior of d1 vs. d2 (Fig. and) and ρd1 vs. ρd2 (Fig. and S10d) provides a clearer picture of the two stages present in Se and Te at HP. Regarding the two mentioned stages of the pre-MMB to MMB transformation in Se and Te, it is worth noting that the behavior of bond distances and bond charge densities in stage 2 of Se and Te is similar to that previously found in stage 2 in pnictogens. The inverse behavior of ρd2 and ρd1 in elemental chalcogens at HP is caused by the trans influence between primary and secondary bonds (see ES in Se Fig. ), as already mentioned for pnictogens in stage 2. As for pnictogens, the trans influence was invoked a long time ago in elemental chalcogens to explain the strong softening of the A1 mode of the A8 phase as well as the increase of the frequencies of the acoustic phonons and the values of the elastic constants in Se-I and Te-I at HP . On the other hand, the normal decrease of the interatomic bond distance and the increase of the bond charge density of MMBs in stage 3 are consistent with the hardening of all phonons in the distorted Ai phase (Fig. and). Again, it is interesting to remark that the pressure range of stage 2 decreases in group-16 elements from Se (0 -23 GPa) to Te (0 -7 GPa); i.e., it decreases for heavier cations, as already observed for group-15 elements. A completely different behavior from pnictogens and chalcogens is found in both phases of Po. The normal, slight decrease (increase) of the bond distance (charge density) in both phases of Po at HP (Fig. ) clearly evidence that both phases of Po do not show several stages at HP. The reason is that they are already in stage 3 at RP because they show fully symmetric MMBs at RP. The pCOHP features of α-Po and β-Po at RP (Fig. ) are very similar and show similar values of IpCOHP(2c) as those found in As-II (Fig. ). Again, the negative value of ICOBI(3c) suggests the presence of ERMBs according to Dronkowski and coworkers . In this context, we must stress that, although the value of ICOBI(3c) in Po is smaller than in As-II, the ratio ICOBI(3c)/IpCOHP(2c) is larger in Po than in As-II. This picture is also consistent with the pressure dependence of the ES (Fig. ) and ELF (Fig. ). Note that the ES≈ 1 for both phases of Po and there is a negligible increase with pressure. This is an expected result for the MMBs in stage 3 since there is no trans influence in this stage and bonds show no gain or loss of charge at HP, just a monotonous decrease (increase) of bond distance (charge density).