A comparison of partial atomic charges for electronically excited states

TL;DR

This paper evaluates various partial atomic charge methods for excited states, demonstrating their effectiveness in interpreting electronic structures and charge transfer processes, especially near conical intersections in conjugated polyenes.

Contribution

It systematically compares partial atomic charge methods for excited states and highlights the iterative Hirshfeld approach as particularly reliable and transferable.

Findings

Iterative Hirshfeld charges align with chemical intuition.

Real-space charges effectively interpret excited-state electronic structures.

Charges are transferable across similar molecular systems.

Abstract

Partial atomic charges are a useful and intuitive concept for understanding molecular properties and chemical reaction mechanisms, showing how changes in molecular geometry can affect the flow of electronic charge within a molecule. However, the use of partial atomic charges remains relatively uncommon in the characterization of excited-state electronic structure. Here we show how well-established partial atomic charge methods perform for interatomic, intermolecular and inter-bond electron transfer in electronically excited states. Our results demonstrate the utility of real-space partial atomic charges for interpreting the electronic structures that arise in excited-state processes. Furthermore, we show how this analysis can be used to demonstrate that analogous electronic structures arise near photochemically relevant conical intersection regions for several conjugated polyenes. Based on our analysis, we find that charges computed using the iterative Hirshfeld approach provide results which are consistent with chemical intuition and are transferable between homologous molecular systems.