Group-16 elements. Now, we present the results of theoretical calculations of the A8 phase of Se (Se-I) and Te (Te-I) as well as of the Ah and Ai phases of α-Po and β-Po, respectively, at different pressures. As expected, the octahedral distortion, measured by the quadratic elongation of the distorted octahedron around each atom of the trigonal A8 phase in Se and Te, decreases as pressure increases (see Fig. and S9c in ESI). However, the relaxation of the A8 structure at HP results in a final octahedrally-coordinated rhombohedral Ai phase instead of the previously assumed cubic Ah phase (see Table in ESI for the structural parameters of the A8 and Ai phases at different pressures). The A8-to-Ai PT is represented in Fig. . Our results agree with the experimental observation of the Ai phase at HP in S-V (above 150 GPa ), in Se-V (above 40-60 GPa ), and also in Te-IV (above 10 GPa ), although the structure of Te-IV has been recently questioned . Therefore, the experimental Pt values for the Ai phase decrease along the series S-Se-Te-Po; a result well reproduced in our calculations for Se (23 GPa), Te (7 GPa), and Po (RP).