The smaller that P σσ cond (r, s), and therefore D σ , are, the more highly localized is the reference electron. This truncated expansion (Eq. 2) is then rescaled over the same type of probability, calculated for a fictitious system having the same density as the system of interest, but behaving like a uniform electron gas, D 0 σ , yielding a dimensionless localization index χ σ given by: