Vertical excitation energies of linear cyanine dyes by spin-flip time-dependent density functional theory

Vertical excitation energies of linear cyanine dyes are examined using the spin-flip time-dependent density functional theory. The Hartree-Fock exchange (HFX) plays an essential role in predicting the absorption spectra, and the best values are obtained by the combination of collinear approximation and hybrid functionals with 50% HFX. The non-collinear approach with pure density functionals underestimates the excitation energy severely. The significant error is due to low excitation energy from the reference triplet to first excited singlet state. The excitation energy decomposition gives small orbital energy difference term and large negative non-collinear kernel. (c) 2015 Elsevier B.V. All rights reserved.