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The band structure of the zinc-blende phase of AlN, GaN, and InN is calculated employing the exactexchange ͑EXX͒ Kohn-Sham density-functional theory and a pseudopotential plane-wave approach. The cation semicore d-electrons are treated both as valence and as core states. The EXX band gaps of AlN and GaN ͑obtained with the Ga 3d electrons included as core states͒ are in excellent agreement with previous EXX results, GW calculations, and experiment. Inclusion of the semicore d-electrons as valence states leads to a large reduction in the EXX band gaps of GaN and InN. Contrary to common belief, the removal of the self-interaction, by the EXX approach, does not account for the large disagreement for the position of the semicore d electrons between the LDA results and experiment.

We have performed a density-functional theory harmonic vibrational analysis of the infinite polyalanine alpha helix. The calculated phonon dispersion spectrum shows excellent agreement to available experimental data, except for the high frequency hydrogen stretching modes which show characteristic shifts due to anharmonic effects. A major advantage compared to previously performed empirical force field studies is that long range effects such as electrostatic interaction and polarization are intrinsically taken into account for characterizing hydrogen bond formation in the helix. Our results indicate that these effects are crucial to accurately describe the low frequency acoustical branches and lead to a significantly better agreement with experiment for the specific heat in the low temperature range.

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