  We calculate the phonon-dispersion relations of several two-dimensional
materials and diamond using the density-functional based tight-binding approach
(DFTB). Our goal is to verify if this numerically efficient method provides
sufficiently accurate phonon frequencies and group velocities to compute
reliable thermoelectric properties. To this end, the results are compared to
available DFT results and experimental data. To quantify the accuracy for a
given band, a descriptor is introduced that summarizes contributions to the
lattice conductivity that are available already in the harmonic approximation.
We find that the DFTB predictions depend strongly on the employed repulsive
pair-potentials, which are an important prerequisite of this method. For
carbon-based materials, accurate pair-potentials are identified and lead to
errors of the descriptor that are of the same order as differences between
different local and semi-local DFT approaches.
