Common errors in BoltzTraP-based calculations

Abstract: Boltzmann transport calculations based on band structures computed from first principles play an important role in modern thermoelectric materials research. Among available codes, the \textsc{BoltzTraP} code is the most widely adopted, but many recent studies contain systematic mistakes. We identify three error modes: (1) inserting the electronic thermal conductivity at zero electric field, $\kappa_0$, in place of the electronic thermal conductivity at zero electric current, $\kappa_e$, (2) computing the figure of merit $zT$ by combining a constant relaxation time of unity while keeping the lattice thermal conductivity $\kappa_\ell$ in standard units, and (3) doing both errors at once. We have found many examples of the third error, but since the first two are simpler, we suspect they are also present in the literature. For the single parabolic band model, we derive exact analytical limits in the non-degenerate and near-degenerate regimes, and we show how mistakes appear for the realistic case study of ZrNiSn. Our results illustrate how faulty calculations can appear reasonable at certain temperatures and Fermi levels, and we provide practical guidance for identifying faulty results and avoiding such pitfalls in thermoelectric transport studies.