Unlike TDDFT, both SF-and MRSF-TDDFT produce a ground singlet (S 0 ) state as one of their response states. The typical ground state of closed-shell systems is mostly described by a single configuration as a result of O2 → O1 spin-flip excitation in Fig. . As the difference of the Kohn-Sham orbital energies is the leading term of the diagonal elements of the A matrix, the vertical excitation energies (VEEs), the energy gaps of S x -S 0 (x ≥ 1) of both SF-and MRSF-TDDFT can be largely proportional to the O2 -O1 gap. The linear relation between them with the amount of the exact exchange contribution is presented in the case of trans-butadiene (See Fig. ), where it is clearly seen that the O2 -O1 gap, as well as VEE, are proportional to the amount of exact exchange. The collinear formalism of MRSF-TDDFT inevitably requires a larger fraction of the exact exchange to produce correct response states, such as in the BH&HLYP functional, which tends to overestimate VEEs. This seemingly contradictory condition may be alleviated by the introduction of an "adaptive exact exchange" (AEE) concept, where individual XC functionals with different amounts of exact exchange are designed for orbital optimization and linear response steps of MRSF-TDDFT calculations. Vertical excitation energies between S 1 and S 0 states with respect to Kohn-Sham orbital energy differences between O2 and O1 for several functionals in trans-butadiene as calculated with MRSF-TDDFT/6-31g(d) basis set. Note that the color represents the amount of exact exchange contribution.