Group-15 elements. The relaxation of the structural parameters of pnictogens (As, Sb, Bi) in the A7 phase at different pressures evidences a decrease in the octahedral distortion of the A7 structure at HP until a final octahedrally-coordinated Ah (sc) structure is reached (see Fig. and Table in ESI). This is evidenced by the decrease of the quadratic elongation of the distorted octahedron around each atom in the A7 structure and the increase of the effective coordination number (see Figs. , and inset of S4a in ESI). In particular, a progressive change in the effective coordination from the trigonally-coordinated A7 structure, characterized by three primary short covalent bonds plus three long secondary bonds (related to LEPs), towards the octahedrally-coordinated Ah structure, characterized by six equal bonds each at 90, is observed. These results are consistent with previous expectations and with the observation of the sc structure in P and As at HP . The best example is As that has an A7-to-Ah phase transition (PT) theoretically predicted at a transition pressure (Pt) of 25 GPa in good agreement with experimental results (see inset of Fig. in ESI) . The theoretically optimized atomic volume and lattice parameters of the As-I (A7) and As-II (Ah) phases at different pressures exhibit a nice agreement with available experimental data and previous calculations (see Fig. in ESI). Remarkably, our structural relaxations of the trigonal A7 phase do not reach the perfect Ah phase except for very high pressures (e.g. our calculations for As just above 25 GPa always show a residual trigonal distortion of the sc phase). This result agrees with the most recent and accurate experimental measurements on phosphorous, where a distorted sc phase (pseudo-sc) is reported as an intermediate phase between the A7 and the Ah phases . A characteristic experimental feature of IV-VI and V2-VI3 chalcogenides, such as GeSe, GeTe, SnTe, and As2S3 that are not PCMs at RP but develop MMBs at HP, is the softening of some optical phonon frequencies at the pre-MMB scenario and hardening of these optical phonon frequencies at the MMB scenario, i.e. a change in sign of the pressure coefficient of the frequency in some optical modes . A similar feature is experimentally found in As and nicely reproduced by our calculations (Fig. ). In this sense, the small values of the frequencies of the Raman-active phonons in As at RP (below 300 cm -1 ) compared to those of Ge (above 300 cm -1 ) and the soft behavior of the optical phonons of As at HP compared to their increase in Ge prove that the bonds in the A7 structure of As are quantitatively and qualitatively different than those of the zinc-blende structure of Ge. This is contrary to expectations since both elements are close in the periodic table and have similar masses, which could suggest similar phonon frequencies and behaviors.