title: On the Metric of Charge Transfer Molecular Excitations: A Simple Chemical Descriptor.
abstract: A new index is defined with the aim of further exploring the metric of excited electronic states in the framework of the time-dependent density functional theory. This descriptor, called Δr, is based on the charge centroids of the orbitals involved in the excitations and can be interpreted in term of the hole-electron distance. The tests carried out on a set of molecules characterized by a significant number of charge-transfer excitations well illustrate its ability in discriminating between short (Δr ≤ 1.5 Å) and long-range (Δr ≥ 2.0 Å) excitations. On the basis of the well-known pitfalls of TD-DFT, its values can be then associated to the functional performances in reproducing different type of transitions and allow for the definition of a "trust radius" for GGA and hybrid functionals. The study of other systems, including some well-known difficult cases for other metric descriptors, gives further evidence of the high discrimination power of the proposed index. The combined use with other density or orbital-based descriptors is finally suggested to have a reliable diagnostic test of TD-DFT transitions.
authors.affiliation: Laboratoire Structures, Propriétés et Modélisation des Solides (SPMS), CNRS UMR 8580, École Centrale Paris , Grande Voie des Vignes, F-92295 Châtenay-Malabry, France.
authors.forename: Ciro A
authors.initials: CA
authors.lastname: Guido
authors.affiliation: Laboratoire Structures, Propriétés et Modélisation des Solides (SPMS), CNRS UMR 8580, École Centrale Paris , Grande Voie des Vignes, F-92295 Châtenay-Malabry, France.
authors.forename: Pietro
authors.initials: P
authors.lastname: Cortona
authors.affiliation: Department of Chemistry, University of Pisa , Via Risorgimento 35, 56126 Pisa, Italy.
authors.forename: Benedetta
authors.initials: B
authors.lastname: Mennucci
authors.affiliation: Laboratoire d'Électrochimie, Chimie des Interfaces et Modélisation pour l'Energie, CNRS UMR-7575, Chimie ParisTech , 11 rue P. et M. Curie, F-75231 Paris Cedex 05, France.|Institut Universitaire de France , 103 Bd Saint-Michel, F-75005 Paris, France.
authors.forename: Carlo
authors.initials: C
authors.lastname: Adamo