Linear Response and Measures of Electron Delocalization in Molecules

The concept of localization and delocalization in molecules is discussed in terms of the response of the electronic system to an external perturbation. It is argued that both the spatial organization of electrons in pairs and the spatial distribution of the response intensity, reflect main features of the correlated motion of electrons, ultimately described by the pair distribution function of electrons. Various measures, derived from the linear charge density response function, are able to characterize localization in a rigorous way, in close analogy to the approach followed in solid state physics.