The DFT optimized structures used for computations in periodic codes (Quantum ESPRESSO and WIEN2k) are given in Fig. , while ORCA utilized the cluster models derived from these structures shown in Fig. . In the periodic DFT calculations the correct electronic structures were confirmed by calculating the electron band structure (see Fig. ), which clearly show conduction bands crossing the Fermi level for the two bandstates and a narrow band gap for the polaron. Afterwards we computed the 2 H EFG tensors and evaluated the quadrupolar parameters, which are given in Table . First we note that for all of the three DFT approaches the polaron and single occupied bandstate quadrupolar parameters are very similar and differentiation between the two experimentally would not be possible, while the double occupied bandstate 2 H quadrupolar parameters are distinct from the other two structures. All of the DFT approaches predict that the double occupied bandstate quadrupolar coupling constant is almost an order of magnitude larger than the experimental value, which indicates that the hydrogen sites host only a single hydrogen ion. However, in the single occupied cases (both bandstate and polaron) there is noticeable discrepancy between the quadrupolar coupling constants obtained with Quantum ESPRESSO and the other two methods. This is most likely due to the different treatment of the electron wavefunctions in each of the software packages. In Quantum Espresso calculations employ plane wave functions to represent the valence electron wavefunction, while the core electrons are approximated with a pseudopotential. WIEN2k