  We present ab-initio phonon dispersion relations for the three lead
chalcogenides PbS, PbSe, and PbTe. The acoustic branches are in very good
agreement with inelastic neutron-scattering data and calculations of the
specific heat give good agreement with experimental data. The pronounced
minimum of the transverse optical branch at Gamma due to the
near-ferroelectricity of the lead chalcogenides is qualitatively reproduced. In
addition, we find a pronounced dip in the longitudinal optical branch at Gamma.
This dip was previously explained as the effect of "free carriers" (due to the
presence of impurities). The calculations demonstrate that it persists also in
the case of pure lead chalcogenides. We explain the dip as a "near Kohn
anomaly" which is associated with the small electronic band-gap at the
high-symmetry point L.
