The package fhi96md is an efficient code to perform density-functional theory totalenergy calculations for materials ranging from insulators to transition metals. The package employs first-principles pseudopotentials, and a plane-wave basis-set. For exchange and correlation both the local density and generalized gradient approximations are implemented. The code has a low storage demand and performs efficiently on low budget personal computers as well as high performance computers. Method of solutionAb initio molecular dynamics on the Born-Oppenheimer surface is implemented by a two step method. In the first step the Kohn-Sham equation [26] is solved self-consistently to obtain the electron ground state and the forces on the nuclei. In a second step these forces are used to integrate the equations of motion for the next time step. The calculation of the total-energy and the Kohn-Sham operator in a plane-wave basis-set is done by the momentum-space method [27].To solve the Kohn-Sham equation the package fhi96md employs the iterative schemes of Williams and Soler [28] and Payne et al. [29]. We use the frozen-core approximation and replace the effect of the core electrons by normconserving pseudopotentials [30][31][32][33] in the fully separable form [34]. The equations of motion of the nuclei are integrated using standard schemes in molecular dynamics such as the Verlet algorithm. Optionally an efficient structure optimization can be performed by a second order algorithm with a damping term. Restrictions on the complexity of the problemOnly pseudopotentials with s-, p-, and d-components are implemented. For highly correlated systems (e.g. f -electrons) the treatment of the exchange-correlation interaction is not appro-2 priate. Relativistic effects are included via scalar relativistic pseudopotentials. The system is assumed to be non-magnetic, but a generalization of the program to magnetic states is straight forward.3 LONG WRITE-UP

The diffusion of intrinsic defects in 3C-SiC is studied using an ab initio method based on density functional theory. The vacancies are shown to migrate on their own sublattice. The carbon splitinterstitials and the two relevant silicon interstitials, namely the tetrahedrally carbon-coordinated interstitial and the 110 -oriented split-interstitial, are found to be by far more mobile than the vacancies. The metastability of the silicon vacancy, which transforms into a vacancy-antisite complex in p-type and compensated material, kinetically suppresses its contribution to diffusion processes. The role of interstitials and vacancies in the self-diffusion is analyzed. Consequences for the dopant diffusion are qualitatively discussed. Our analysis emphasizes the relevance of mechanisms based on silicon and carbon interstitials.

The signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many-body perturbation theory. A single BN-sheet serves as a model for different layered BN nanostructures and crystals. In the sheet we embed prototypical defects such as a substitutional impurity, isolated boron and nitrogen vacancies, and the divacancy. Transitions between the deep defect levels and extended states produce characteristic excitation bands that should be responsible for the emission band around 4 eV, observed in luminescence experiments. In addition, defect bound excitons occur that are consistently treated in our ab initio approach along with the "free" exciton. For defects in strong concentration, the coexistence of both bound and free excitons adds substructure to the main exciton peak and provides an explanation for the corresponding feature in cathodo-and photoluminescence spectra.