Double Excitations in Molecules Using Screened Configuration Interaction.

Area of Science:

• Computational chemistry
• Quantum mechanics
• Molecular physics

Background:

• Double excitations are vital for understanding singlet fission and photophysics in organic molecules.
• Accurate theoretical descriptions of photoexcited states with mixed single and double excitation character are needed.

Purpose of the Study:

• To develop and apply first-principles many-body theory methods for describing photoexcited states with both single and double excitation character.
• To evaluate the performance of two novel configuration interaction (CI)-based methods: screened configuration interaction singles and doubles (scrCISD) and screened configuration interaction singles with perturbative doubles (scrCIS(D)).

Main Methods:

• Employed a first-principles many-body theory framework combining GW/Bethe-Salpeter equation and CI methods.
• Developed and applied scrCISD and scrCIS(D) methods to an effective many-body Hamiltonian incorporating screening.
• Applied methods to Thiel's set of molecules and pentacene dimers.

Main Results:

• The scrCISD method systematically underestimated excitation energies.
• The scrCIS(D) method demonstrated good agreement with established theoretical estimates for excitation energies.
• Calculated binding energies for doubly excited triplet pair states (TT¹) in pentacene dimers using scrCISD, showing good agreement with empirical data.

Conclusions:

• The scrCIS(D) method provides an accurate and reliable approach for calculating excitation energies in organic molecules with significant double excitation character.
• The developed methods offer valuable tools for investigating complex photophysical processes like singlet fission.