Exploring excited states using Time Dependent Density Functional Theory and density-based indexes

The recent advances in the development and application of density-based indexes for the description of the nature and the quantification of the extent of charge transfer associated with a given electronic transition are here reviewed. Starting from the basic definition of the indexes, a brief overview of their potential as indicators of potentially problematic cases in the description of charge transfer excitations using Time Dependent Density Functional Theory (TD-DFT) will be first given together with their possible application for comparing TD-DFT results to post Hartree-Fock (post-HF) calculations. After this methodological part, several examples of the application of density-based indexes to describe, from a quantitative and qualitative point of view, the charge transfer character (for instance in push pull systems) or to map excited state reaction pathways (for instance in the case of Excited State Proton Transfer reactions) will be given to exemplify the insights that these indexes may bring to the description and design of new compounds of potential technological relevance.