The derivatives of the SOCMEs with respect to the nuclear coordinates have been calculated numerically using a three-point finite-difference approximation with a step length of 0.001 Å. Considering the three degenerate electronic substates of the triplet (m s = 0, ±1), the calculated RISC rate is scaled by a factor of 1/3. Moreover, the reported NAC values are calculated by taking norms of NAC vectors at three directions for individual atoms and then summing them up. nm (2.67 eV), respectively, with a Stokes shift of 18 nm. The calculated vertical and adiabatic excitation energies (VEEs and AEEs) by TDA, full TDDFT, and ADC(2) methods are shown in Table . It is clear that TDDFT with and without TDA slightly overestimates the absorption and emission energies, and significantly overestimate the energy gap of ∆E S 1 T 1 compared to the experimental values of 1532 cm -1 (0.19 eV). More specifically, the first three functionals (B3LYP, CAM-B3LYP*, and ωB97XD*) in Table produce the vertical and adiabatic energy gaps ∆E S 1 T 1 ≥ 0.5 eV. The CAM-B3LYP* TDDFT calculations, for instance, predict vertical excitation energy of 2.87 and 2.36 eV for S 1 and T 1 states, respec-tively. While the LC-BLYP* yields a slightly smaller ∆E S 1 T 1 value of ∼ 0.4 eV for vertical excitation and 0.3 eV for adiabatic transition. The appreciable overestimation to ∆E S 1 T 1 by the TDDFT approach indicates that 7-PhQAD molecule is a strongly correlated system. To check this, we calculate VEEs of low-lying excited states at the theoretical level of ADC(2)/cc-pVDZ. The ADC methods involve a perturbative expansion of a propagator followed by truncation at a certain order n that defines the ADC(n) approximation. As concluded by previous studies on the MRtype TADF materials, the coupled-cluster (CC) family such as CC2 and CCSD or ADC approach such as ADC(2) could take double excitations or second-order dynamic electron correlation effect into account and therefore, reproduce more accurate S 1 -T 1 energy gap than TDDFT does. Even though ADC(2)/cc-pVDZ overestimates the absorption and emission energies (see Table in SI), it produces a S 1 -T 1 energy gap of 0.27 eV, which is much closer to the experimental measured value 0.19 eV. The second-order correction contributes 13% and 11% to S 1 and T 1 states, respectively, as Table in SI shows, meaning that the failure of TDDFT approach is owing to the absence of an explicit account of double (or higher order) excitations. To exactly describe 7-PhQAD molecule, one thus has to properly account for electron correlation. T 1 states have similar hole and particle orbitals, the SOCME between these two states is deduced to be small, which is actually verified by the calculated values shown in Table .