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When the values of the off-diagonal elements <math>U_{\bold{G}}</math> between the reciprocal lattice vectors in the Hamiltonian almost go to zero. As a result the magnitude of the band gap <math>2|U_{\bold{G}}|</math> collapses and the empty lattice approximation is obtained.
==The
Apart from a few exotic exceptions, [[metal]]s crystallize in three kinds of crystal structures: the BCC and FCC [[cubic crystal system|cubic crystal structures]] and the [[hexagonal crystal system|hexagonal]] close-packed [[close-packing of spheres#fcc and hcp lattices|HCP]] crystal structure.
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