Regarding the vibrational properties, both Se and Te exhibit a considerable experimental softening of the A1 mode of the A8 phase, whose frequency pressure coefficient changes above ca. 18 and 7 GPa, respectively . These results are well reproduced by our simulations, except near Pt, where our simulations predict a second-order PT, similar to pnictogens. Again, this disagreement is likely caused by the lack of anharmonic interactions (that seem to be only significant near Pt) in our simulations. The Ai phase does not exhibit optical Raman-active modes (only acoustic phonon modes), but our calculations provide non-zero optical phonon frequencies above 23 GPa in Se and Te (Fig. and) due to the remaining distortion concerning the trigonal Ai phase. All the calculated optical phonon frequencies in the distorted Ai phase of Se and Te harden with increasing pressure, as expected once MMBs are formed. As for group-15 elements, the formation of MMBs in the Ai phases of Se and Te is confirmed by the high values of Z* and of the |γi| for the A1 mode of the A8 phase (Fig. and) as well as by the negligible value of ∇ 2 ρ (Fig. and). Hence, we conclude that MMBs are formed in Se and Te at HP as they approach the octahedral coordination of the Ai phase.