If one wants to include in a theoretical model more details of the crystal structure itself, then instead of an ideal crystal approximation (Fig. ), which directly approximates the Fermi surface of a metal to a sphere (which is valid for alkaline metals mostly), one should introduce the periodicity via the Kronig-Penney model (see Fig. ). Firstly, this model was introduced as a periodic lattice of rectangular potentials. However, there exists a simpler model (i.e. with fewer free parameters) called Dirac-Kronig-Penney (DKP) model i.e. instead of the rectangular quantum wells, we have the Dirac potentials, as below