sentences:  Wigner–Seitz radius
  
  The Wigner–Seitz radius formula_1, named after Eugene Wigner and Frederick Seitz, is the radius of a sphere whose volume is equal to the mean volume per atom in a solid (for first group metals).In the more general case of metals having more valence electrons, formula_1 is the radius of a sphere whose volume is equal to the volume per a free electron.