Ab initio optical properties of BN in the cubic and in the layered hexagonal phase

We present our ab initio calculation of linear optical properties in cubic (c-BN) and layered hexagonal BN (h-BN). Our results show a reasonable agreement with existent theoretical results and with measured data for the imaginary part of the dielectric function in the case of h-BN, while for c-BN discrepancies arise between theory and experiment. We discuss the possible reasons of this mismatch.