English: Illustration of tight binding approximation. The black filled marks represent the atomic sites. The green colored functions are the wave functions of the atom at each atom site $\psi(r)$. The overlap between the wave functions of two neighboting atoms is not neglectable when the distance is close enough. The light gray, dashed line represents the atomic potential $V_a(r)$, while the drak gray continuous line represents the rest potential $\Delta V(r)$ resulting from the overlaping potentials of neigboring atoms.