Views of the mode 1 dispersion surface in the QxQy plane of the reciprocal lattice of aluminium, using the original Lennard-Jones potential (before fitting has been applied), with scattering intensity superimposed upon the frequency surface as a colour map. (a) An isometric projection of the dispersion surface. (b) A cross section through the plane from the Qx-axis perspective. (c) The view looking down on the plane. (d) A diagrammatic representation of the (hk0) plane in aluminium, showing for example the  direction (broken line). The two spheres in Q show the upper and lower values of Q for which one-phonon scattering can occur as a result of the condition |τ − q| < Q < |τ + q|.