  Linear optical functions of cubic and hexagonal BN have been studied within
first principles DFT-LDA theory. Calculated energy-loss functions compare well
with experiments and previous theoretical results both for h-BN and for c-BN.
Discrepancies arise between theoretical results and experiments in the
imaginary part of the dielectric function for c-BN. Possible explanation to
this mismatch are proposed and evaluated; lattice constant variations, h-BN
contamination in c-BN samples and self-energy effects.
