Extended Data Fig. 5: Band structure.

Comparison of the band structure at 260 GPa for the average centroid position with different DFT exchange-correlation functionals. TB09 is the functional we used for computing optical properties in the main text. Since the structure is very close to a hexagonal cell, we adopt the typical k-path along high symmetry lines for hexagonal crystals. Both HSE06 and TB09 go beyond DFT, employing, respectively, hybrid and meta-GGA functionals. HSE06 is a general-purpose functional that provide very good electronic bands, at a much high computational cost required by the calculation of the exact exchange. On the other side, TB09 is aimed only to compute electronic properties, present a lot of limitations on isolated atoms, but it has a computational cost comparable to that one of BLYP (allowing calculation with more atoms in the cell). The precision of TB09 and HSE06 in the band structure calculation is comparable. By assuming a similar accuracy of TB09 and HSE06, we can estimate an error of 0.4 eV in the bandgap of each functional on static configurations.