title: Charge-Transfer Versus Charge-Transfer-Like Excitations Revisited.
abstract: Criteria to assess charge-transfer (CT) and CT-like character of electronic excitations are examined. Time-dependent density functional theory (TDDFT) calculations with non-hybrid, hybrid, and tuned long-range corrected (LC) functionals are compared with coupled-cluster (CC) benchmarks. The test set comprises an organic CT complex, two push-pull donor-acceptor chromophores, a cyanine dye, and several polycyclic aromatic hydrocarbons. Proper CT is easily identified. Excitations with significant density changes upon excitation within regions of close spatial proximity can also be diagnosed. For such excitations, the use of LC functionals in TDDFT sometimes leads to dramatic improvements of the singlet energies, similar to proper CT. It is shown that such CT-like excitations do not have the characteristics of physical charge transfer, and improvements with LC functionals may not be obtained for the right reasons. The TDDFT triplet excitation energies are underestimated for all systems, often severely. For the CT-like candidates, the singlet-triplet (S/T) separation changes from negative with a non-hybrid functional to positive with a tuned LC functional. For the cyanine, the S/T separation is systematically too large with TDDFT, leading to better error compensation for the singlet energy with a non-hybrid functional.
authors.affiliation: Department of Chemistry, University at Buffalo, State University of New York, Buffalo , New York 14260-3000, United States.
authors.forename: Barry
authors.initials: B
authors.lastname: Moore
authors.affiliation: Department of Chemistry, University at Buffalo, State University of New York, Buffalo , New York 14260-3000, United States.|State Key Laboratory of Precision Spectroscopy, Department of Physics, East China Normal University , Shanghai 200062, P. R. China.
authors.forename: Haitao
authors.initials: H
authors.lastname: Sun
authors.affiliation: Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory , Richland, Washington 99352, United States.
authors.forename: Niranjan
authors.initials: N
authors.lastname: Govind
authors.affiliation: Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory , Richland, Washington 99352, United States.
authors.forename: Karol
authors.initials: K
authors.lastname: Kowalski
authors.affiliation: Department of Chemistry, University at Buffalo, State University of New York, Buffalo , New York 14260-3000, United States.
authors.forename: Jochen
authors.initials: J
authors.lastname: Autschbach