The theoretically optimized atomic volume and lattice parameters of the A8 and Ai phases of Se and Te at different pressures exhibit a nice agree with available experimental data and previous calculations (see Fig. and S9 in ESI) . A progressive change is observed in the effective atomic coordination from the twofold-coordinated A8 structure, characterized by two primary short covalent p-type bonds plus four long secondary bonds (related to LEPs), towards the octahedrally-coordinated Ai structure of -Po, characterized by six equal bonds. It must be noted that our structural relaxations of the trigonal A8 phase do not reach the Ai phase of Po at RP, except for very high pressures, so our calculations always show a residual bond distortion of the Ai structure in the pressure range discussed here (see data in Table of ESI).