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Sometimes the electrons aren't free, but are basically bound
to the atoms of the lattice. By perturbing the electron
wave functions in terms of the boung hydrogenic wave functions,
one can apply a perturbative expansion to this
Tightly-Bound scenario.
The energy structure has a much different appearance, as each
of the atoms has a degeneracy which is lifted. For a chunk of
everyday-sized material, there are usually of order 1023
atoms, all of which are degenerate. The lifting of the degeneracy
spreads these states out into a quasi-continuous
energy band.
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