Wannier functions have recently found application in describing the polarization in crystals, for example, ferroelectrics. The modern theory of polarization is pioneered by Raffaele Resta and David Vanderbilt. See for example, Berghold, and Nakhmanson, and a power-point introduction by Vanderbilt. The polarization per unit cell in a solid can be defined as the dipole moment of the Wannier charge density: