Energy is the prevailing criterion for a first assessment of the crystal structures. From the total electronic energies (E tot ) the cohesive energies were obtained by deducting the atomic contributions. The resulting atom cohesive energies of three BP polymorphs were then compared. The results provided in Table show close atom average cohesive energies with a slightly more stable rhombohedral form. This validates the choice of rh-ZnS as the starting model for the structural refinement of rh-BP.