with ℏ ℏ the Planck constant, m e the electron mass and ω the characteristic frequency of the oscillations. The latter can be estimated by considering the electrostatic potential energy for an electron displaced by r from its lattice point. Say that the Wigner–Seitz cell associated to the lattice point is approximately a sphere of radius a /2. The uniform, neutralizing background then gives rise to a smeared positive charge of density 6 e / a 3 π π with e the electron charge. The electric potential felt by the displaced electron as a result of this is given by