However, that was not the only result obtained by Lang and Kohn, as they also obtained the electron-charge-density profiles in two important cases: (1) simply due to the existence of the surface (being the impurity of the greatest contribution here), as depicted on the Fig. , and (2) due to the uniform external electric field applied as on the Fig. -the Wigner (or Wigner-Seitz) radius rs is the parameter describing the electron density of the given metal in a bulk form and is defined as 116 rs = 3V 4πN