Charge-Transfer Versus Charge-Transfer-Like Excitations Revisited

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurately characterizing electronic excitations, particularly charge-transfer (CT) phenomena, is crucial in computational chemistry.
• Time-dependent density functional theory (TDDFT) is a widely used method, but its performance with different functionals for CT and CT-like excitations requires careful evaluation.

Purpose of the Study:

• To establish reliable criteria for assessing charge-transfer (CT) and CT-like character in electronic excitations.
• To compare the performance of various TDDFT functionals (non-hybrid, hybrid, LC) against coupled-cluster (CC) benchmarks for different molecular systems.

Main Methods:

• Utilized time-dependent density functional theory (TDDFT) with a range of functionals: non-hybrid, hybrid, and tuned long-range corrected (LC).
• Employed coupled-cluster (CC) methods as high-level benchmarks for comparison.
• Investigated a diverse test set including an organic CT complex, donor-acceptor chromophores, a cyanine dye, and polycyclic aromatic hydrocarbons.

Main Results:

• Proper charge-transfer (CT) excitations were readily identified.
• TDDFT with LC functionals sometimes showed significant improvements for CT-like excitations, but these gains may not reflect true physical charge transfer.
• TDDFT consistently underestimated triplet excitation energies across all tested systems.

Conclusions:

• While TDDFT can identify true CT, caution is advised for CT-like excitations, as improvements with LC functionals may be artifacts.
• The singlet-triplet (S/T) energy separation is sensitive to functional choice, with implications for accuracy in different systems.
• Non-hybrid functionals may offer better error compensation for singlet energies in certain cases, like cyanine dyes, despite issues with S/T separation.