Vibrational spectrum of the (Al₂O₃)_n, n = 1–4, ground-state clusters and their corresponding structures. The structures (for n = 1–4) are depicted with increasing n from left to right and from top to bottom. The black line represents a Lorentzian distribution with HWHM parameter γ = 0.2 for all peaks. The left panel shows the intensity in units of km/mole; the right panel the opacity in units of cm²/g.