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In the framework of density functional
theory (DFT), the lowest
triplet excited state (T1) can be evaluated using multiple
formulations, the most straightforward of which are unrestricted density
functional theory (UDFT) and time-dependent density functional theory
(TDDFT). Assuming the exact exchange–correlation (XC) functional
is applied, UDFT and TDDFT provide identical energies for T1 (ET), which is also a constraint that
we require our XC functionals to obey. However, this condition is
not satisfied by most of the popular XC functionals, leading to inaccurate
predictions of low-lying, spectroscopically and photochemically important
excited states, such as T1 and the lowest singlet excited
state (S1). Inspired by the optimal tuning strategy for
frontier orbital energies [T. Stein, L. Kronik, and R. Baer, J. Am. Chem. Soc. 2009, 131, 2818], we proposed a novel and nonempirical prescription
of constructing an XC functional in which the agreement between UDFT
and TDDFT in ET is strictly enforced.
Referred to as “triplet tuning”, our procedure allows
us to formulate the XC functional on a case-by-case basis, using the
molecular structure as the exclusive input, without fitting to any
experimental data. The first triplet tuned XC functional, TT-ωPBEh,
is formulated as a long-range-corrected (LRC) hybrid of Perdew–Burke–Ernzerhof
(PBE) and Hartree–Fock (HF) functionals [M. A. Rohrdanz, K.
M. Martins, and J. M. Herbert, J. Chem. Phys. 2009, 130, 054112] and tested on four sets
of large organic molecules. Compared to existing functionals, TT-ωPBEh
manages to provide more accurate predictions for key spectroscopic
and photochemical observables, including but not limited to ET, the optical band gap (ES), the singlet–triplet gap (ΔEST), and the vertical ionization potential (I⊥), as it adjusts the effective electron–hole
interactions to arrive at the correct excitation energies. This promising
triplet tuning scheme can be applied to a broad range of systems that
were notorious in DFT for being extremely challenging
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