One of the most fruitful methods developed for solving the many-electron problems in a crystal is the one-electron approximation. In this method the total wave functions of electrons are chosen as a linear combination of the individual wave functions in which each wave function involves only the coordinates of one electron. It is this approximation that forms the basic framework for calculating the energy band structure of a solid. This method can be described by assuming that each electron sees, in addition to the potential of the fixed charges (i.e., positive ions), only some average potential due to the charge distribution of the rest of the electrons in the solid. Therefore, the movement of each electron is essentially independent of the other electrons throughout the crystal lattice. By means of the one-electron approximation, the solution of the many-electron problems is reduced to: (1) finding equations which are satisfied by the one-electron wave functions, and (2) obtaining adequate solutions for the electron wave functions and electron energies in the crystal under consideration.