Above, the length of the sample is denoted by L . We now assume that our solid is a periodic crystal with each unit cell labeled by a lattice position vector R → → . Position within a unit cell is given by a vector x → → such that the overall position in the crystal may be expressed as r → → = R → → + x → → . Because of the translational invariance of our unit cells, the potential distribution of every cell is identical and V ( x → → ) = V ( x → → + R → → ) .