Experimentally, a sign change in the pressure coefficients of the optical phonon frequencies occurs at 25, 9, and 3 GPa in As, Sb, and Bi (see Fig. and S4c in ESI for Sb and Bi) . These results indicate that MMBs are experimentally formed above such pressures. Our simulated Pt values for the A7-to-Ah PT in pnictogens in fact agree for As and Sb (7 GPa), but not for Bi (14 GPa). Since the Pt value is expected to decrease along the series P-As-Sb-Bi, in agreement with recent experimental results for phosphorous , the larger value for Bi than for Sb suggests that PBEsol functional does not work well for Bi. This has been confirmed by lattice dynamics calculations of Bi-I that seem to work well up to 6 GPa, but not above this pressure (Fig. in ESI). The reason is the lattice dynamical instability of the the A7 structure above 6 GPa suggested by the presence of imaginary phonons (Fig. ). Better calculations for Bi-I with the AM05 functional (pink line in Fig. ), which was already used for Bi2S3 , show that compression of the A7 structure results in a monoclinic phase above 2.0 GPa that fully agrees with the experimental observation of the monoclinic Bi-II phase around 2.5 GPa . In any case, we show hereafter the results obtained with the PBEsol functional for Bi for the sake of comparison with As and Sb under similar conditions.