Moreover, the Thomas-Fermi approximation is applicable only in terms of the long-wavelength charge variations. The common understanding of metals in bulk is that the material experiences only a minor change due to the presence of a defect (perturbation). Anything that perturbs the ordered structure inside a crystal may be considered a defect -the presence of an adatom/vacancy/substituted atom and so on. Interestingly, even a surface itself is a defect because the atoms at the surface have a different environment from the atoms in the bulk of the crystal. This, however, also means that anything affecting the electron state of the surface (later termed as simply "surface state") can also be considered a defect, like an atom atop an otherwise ideal surface or even a charged particle that approaches the said surface.