where rj (t), and rj (t + t ′ ) denote the unit vectors connecting from the center of mass of anions to an edge atom in the same anion at time t and t + t ′ , respectively. N e is the number of edge atoms in the anion. This function typically shows an exponential decay (≈ e -λt ′ ) within a short time with a decay rate λ. The anion reorientational activation energy (E r a ) also can be calculated using the Arrhenius equation: