It has been recognized around 2019 that the very popular time-dependent density functional theory (TDDFT) is not suitable for the calculation of singlet-triplet gaps of organic chromophores due to the lack of double excitations. The latter systematically stabilize the singlet state relative to the triplet state of HOMO-LUMO excitation character. Computationally efficient single-reference wavefunction methods, such as CC2 (second-order simplified coupled-cluster method) or ADC(2) (algebraic-diagrammatic construction of second order) were established as reliable methods for the calculation of S1 and T1 excitation energies by benchmarking these methods with CC3 ( third-order simplified coupled-cluster method) or other accurate electronic-structure methods. Meanwhile, it has been shown that the TD-DFT method with suitable double-hybrid functionals also can provide reliable singlet-triplet gaps. In the present work, the ADC(2) and CC2 methods were employed.