Abstract: The dispersion relation of a diatomic chain has two branches. For a diatomic chain with on-site potential, both these modes are optical ones, because there is also a natural gap for small wave numbers in the lower branch of dispersion relation. In the case of long wavelength ＂acoustic＂ modes, the amplitudes of the two atoms within each cell are not identical. The long wavelength ＂acoustic＂ modes are not the motions of the center of mass.

Key words: on-site potential, diatomic chain, oscillation image