where C is the number of migrating species per unit volume. Therefore, the cross-correlation between migrating ions can be quantified from the ratio of D * and D J , which is called the Haven's ratio (H R ) . Note that H R does not measure the correlation between subsequent hops of a single ion that is migrating, i.e., the deviation of the trajectory of a single migrating ion from a fully random walk. This deviation from a fully random walk is measured by the correlation factor ( f ) of Eq. 7.