Computational Materials play a central role in modern materials research and technology. They can provide information not straightforwardly accessible by experiment and they can identify the dominant mechanisms governing the properties of materials. In this seminar, a short introduction to first-principles calculations, which nowadays constitute the working horse in the field of Computational Materials research, will be provided. Surfaces and interfaces, which are the key features that control both the properties as well as the growth of nanomaterials, will be used to highlight how these calculations can provide an atomic scale understanding of the physics and reveal the dominant mechanisms of semiconductor nanostructures. Specifically, we will examine and discuss the formation and properties of nanopipes and V-pits in and on III-Nitrides and elaborate on their influence on the optoelectronic properties of these materials.