A simple measure of electron localization in atomic and molecular systems

We introduce in this work a new approach to the identification of localized electronic groups in atomic and molecular systems. Our approach is based on local behavior of the Hartree-Fock parallel-spin pair probability and is completely independent of unitary orbital transformations. We derive a simple ``electron localization function'' (ELF) which easily reveals atomic shell structure and core, binding, and lone electron pairs in simple molecular systems as well.