In pristine semiconductors and insulators many electronic transitions are either forbidden or very weak because of selection rules imposed by the high symmetry of the solid. In many ways point defects (impurities, vacancies, interstitials, and various complexes) can be thought of systems with broken symmetry where formerly forbidden transitions become allowed. Depending on the application of the material, these new transitions can be either beneficial, neutral, or detrimental. The example of beneficial properties are defects that are currently investigated in view of their application in quantum information processing and nano-metrology. The example of detrimental properties are defects that serve as nonradiative recombination centers in optoelectronic devices. In both cases, defects affect the properties of materials dramatically.

Over the past decade computational techniques to study defects in solids have finally reached the level where a direct comparison with experiment can be made. Moreover, these techniques can provide the microscopic understanding that can complement and guide the experimental work. In this talk an overview of some of the author’s recent work on the electronic structure of point defects will be given. In the beneficial domain (defects for quantum information processing and nano-metrology), electron-phonon coupling in nitrogen-vacancy and silicon-vacancy centers in diamond will be discussed. In the detrimental domain, nonradiative transitions at defects in group-III nitrides will be analysed and the implications of these findings for efficiencies of nitride light-emitting diodes will be highlighted.