To check the stability of different configurations of rubidium on the three crystallographically different alkali metal positions (see Figure ) according to K0.67Rb0.33Tl, the ground state energies for three situations were calculated (see Figure and SI chapter 11). This clearly shows that the favored position for rubidium is A1. This result is in excellent agreement with the observations of the s.o.f. from the experimental data (see Table ).