Electron correlation decides the stability of cubic versus hexagonal boron nitride

Periodic local Moller-Plesset second-order perturbation theory (MP2) is applied to investigate the structural and energetic properties of the cubic and hexagonal polymorphs of boron nitride. While the Hartree-Fock (HF) solution significantly underbinds both systems and energetically favors h-BN, the post-HF correlation treatment recovers the lacking amount of the interaction energy and reverts the sign of the relative stability between the two compounds. It provides the physically correct picture and predicts cohesive energies, lattice constants, bulk moduli, and the relative stability in good agreement with experiment. Density-functional theory (DFT) results, on the other hand, are inconclusive and exhibit a strong dependence on the chosen functional. The results of MP2 as well as DFT with an empirical dispersion correction indicate that the dispersion contribution to binding is essential not only for the layered polymorph, but also (and even more so) for the cubic one.