The Peierls instability: a 1D electron gas has a divergent susceptibility at wavevector Q = 2k_F — perfect Fermi surface "nesting". This drives a charge density wave ρ(x) = ρ₀[1 + Δcos(2k_F x + φ)] and opens a gap at k=±k_F. The CDW condenses below T_CDW (often 100–300 K) in NbSe₂, TaS₂, and NbTe₂. The Lindhard function χ(q) diverges at q=2k_F (red peak), signaling instability.