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The scattering potential of the crystal is given by,
 [2205a]
where,
τ_{n}  The atomic coordinate for the n^{th} atom,
f_{q}  The atomic scattering factor,
V_{c}  The volume of the supercell,
ħq  The momentum transfer.
In crystals, a periodic potential appears because the ions are arranged with a periodicity of their Bravais lattice. The periodic potential can be given by,
U(r + R) = U(r)  [2205b]
where,
R  The lattice vector.
This potential can be applied into the Schrödinger equation,
 [2205c]
In these cases, the electrons in the crystals are no longer free electrons, but are now called Bloch electrons.
