As already reported by other theoretical works, it seems that in the cubic phase sodium will prefer to sit within the face centered interstitial site (Wyckoff position 24d) instead of occupying the body centered void (Wyckoff position 8c). The exact energetic behaviour of this phenomenon and an extended structural comparison can be found in Figure and Table . Interestingly, the above presented structural changes in R-PW lead to the structure of the face centered interstitial site being stretched in one direction, while being compressed in the other. This change significantly reduces the space available to the sodium ion by 30%, causing a migration away from the fcc plane into the vertices of the cubes during the phase transition. The full account of these geometric factors can be found in Figure and Table . We tried to also energetically compare the equivalents of the interstitial positions known from the cubic phase with their equivalents in R-PW, but these positions are -despite extensive efforts -not stable enough to converge within our set convergence criteria. Instead, after the optimization process the sodium ions always revert back to their minimum energy positions.