where p represents the population density function (the occupancy) of the cations, p max is the maximum value of the population density function, ∆V is the difference in potential energy of the individual cations (eq. ( )), and ∆S is the difference in entropy (the relative entropy). Finally, we calculated the "self" part of the self-van Hove correlation function, G S (r, t ), to understand the Na + ion hopping mechanism using the formula ,