Nothing in the world is perfect. This is also true in materials research. In computer simulations, one often represents a system in a highly idealized way; for example, one calculates the properties that an absolutely perfect crystal would have. In practice, however, we always have to deal with additional effects—with defects in the crystal lattice, with additional particles that attach to the material, with complicated interactions between the particles. The crucial question is therefore: Do these unavoidable additional effects change the material properties or not?