Two Classical Limits—Tight Binding and Nearly Free Electron

Two Classical Limits—Tight Binding and Nearly Free Electron 

In reality, typical solids of interest occupy a spectrum between these two extremes, from the alkali halides, which are well described by TB theories, to Al, which is well described by NFE theories. (It is not, in fact, straightforward NFE theory, but rather pseudopotential theory, which is successful for Al, but for the moment this distinction will be ignored.) Intermediate between the two cases are, for instance, group IV semiconductors, in which both points of view have proved useful, and transition metals, in which different parts of the band structure conform to NFE and TB descriptions. 

In describing NFE and TB theory, the emphasis will be on model calculations which convey the essence of these points of view, rather than practical detail. 

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