Fermi level and electron counting

An important point we must consider now is how these crystal orbitals are fiUed with electrons. Usually, the occupancy of the energy levels of a band is discussed in terms of the number of electrons per unit cell. As a band contains N different 

1) All along this chapter we refer to the Fermi level as the highest occupied energy level, that is, we use it just to count electrons. Note, however, that in the case of a semiconductor, the thermodynamic definition of the Fermi level places it within the energy gap [6], 

R and k vectors have two or three components, respectively, and, consequently, 

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