The Peierls instability (1939)

Competition:  Electronic energy vs. Lattice (elastic) energy (recall that band electrons move in a weak, periodic potential)

Electronic states moves lower in energy   ~ D2  ln(t/D)

Elastic energy is increased  ~ D2

At low temperature dimerization always favorable

kFermi is still  p/2a  and since d = 2p/k , d = 4a   (charge density wave)