Using Density Based Indexes and Wave Function Methods for the Description of Excited States: Excited State Proton...

To provide tools to interpret photochemical reactions, in this paper we demonstrate how a recently developed density-based index (D_CT), up to now used in conjunction with time dependent density functional theory methods, can be extended to multiconfigurational methods. This index can guide chemists in the interpretation of photochemical reactions providing a measure of the spatial extent of a photoinduced charge transfer and, more generally, of charge transfer phenomena. This qualitative and quantitative description can be particularly relevant in the case of multiconfigurational calculations providing a simple tool for the interpretation of their complex outputs. To prove the potentiality of this approach we have considered a simple intramolecular excited state proton transfer reaction as study case and applied both wave function (CASSCF-CASPT2) and density-based methods in conjunction with a D_CT analysis. Our results confirm that, also in the case of multiconfigurational methods, the D_CT provides very useful information about the structural reorganization of a molecule at the excited state.