Our theoretical results for Se and Te can be compared with experiments. In particular, several soft phonons still appear in the Se-II and Se-III phases (below 28 GPa) as well as the Te-II phase (below 8 GPa) and similar soft phonons have been observed in amorphous chalcogenides related to PCMs. Since the amorphous phases in chalcogenides, such as GST, are characterized by a mixture of conventional and unconventional bonds with geometries similar to those of the hypercoordinated multicenter units of Fig. -b according to the VSEPR theory , we also consider that the above-mentioned phases of Se and Te are likely characterized by a mixture of covalent bonds and MMBs with geometries similar to those of Fig. . Further justification of the geometries described in Fig. will be given at the end of section 4. The hardening of optical phonons in Se occurs experimentally between 18 and 28 GPa (Fig. ), i.e. upon transition from Se-II to Se-III, and especially to the Se-IV phase . This change is related to the decrease of resistivity in this pressure region . Similarly, the hardening of optical phonons in Te occurs experimentally above 7 GPa ; i.e. upon transition to Te-III , which is also related to the decrease in resistivity . These changes in Se and Te provide additional support to the presence of asymmetric MMBs in Se-IV (Te-III) as predicted by our calculations above 23 GPa. It must be stressed that the presence of asymmetric MMBs in these intermediate phases of Se and Te, as already suggested to be present in Bi-II, likely occurs before reaching the perfect Ai structure of Se-V and Te-IV phases that are experimentally found at slightly higher pressures.