Double excitations in molecules

TL;DR

This paper develops and applies advanced many-body computational methods to accurately describe double excitations in organic molecules, crucial for understanding photophysical processes like singlet fission.

Contribution

It introduces and tests two CI-based methods, scrCISD and scrCIS(D), incorporating screening effects to improve excitation energy predictions in molecules.

Findings

scrCIS(D) shows good agreement with theoretical estimates

scrCISD systematically underestimates excitation energies

Calculated binding energies of triplet pair states match empirical data

Abstract

Double excitations in organic molecules have garnered significant interest as a result of their importance in singlet fission and photophysics. These excitations play a crucial role in understanding the photoexcitation processes in polyenes. To describe photoexcited states with both single and double excitation character, we use a first-principles many-body theory that combines the GW / Bethe-Salpeter equation and the configuration interaction (CI) methods. Specifically, we develop and employ two CI-based methods: screened configuration interaction singles and doubles (scrCISD) and screened configuration interaction singles with perturbative doubles (scrCIS(D)), applied to an effective many-body Hamiltonian that incorporates screening. We apply these methods to Thiel's set of molecules, which exhibit excited states predominantly characterized by single excitations with a partial double excitation character. Our results indicate that the scrCISD method systematically underestimates the excitation energies compared to the best theoretical estimates, while the scrCIS(D) method shows good agreement with these estimates. Furthermore, we used the scrCISD method to calculate the binding energies of the dominantly doubly excited correlated triplet pair states, $\mathrm{TT^1}$, in pentacene dimers, finding that the $\mathrm{TT^1}$ binding energies agree well with empirical calculations.