Computational dye design by changing the conjugation order: Failure of LR-TDDFT to predict relative excitation energies in organic dyes differing by the position of the methine unit

► Large differences in spectra of dyes differing by conjugation sequence are analyzed. ► (TD)DFT errors are more important vs. dyes differing by size of conjugation. ► TDDFT errors due to long-range are corrected using a measure of charge-transfer. ► Traditional functionals + correction better than range-separated functionals. ► HOMO-LUMO gap is a good predictor of relative energies even with long-range effects.