Due to the small energy differences between the low-lying triplet states and larger SOCMEs between S 1 and T n (n = 2, 3) than that between S 1 and T 1 , the contribution of SVC effect to the rates of S 1 -T 1 crossings is roughly one order of magnitude larger than that of HTVC effect. For example, at ADC(2) level, the excitation energy differences between T 2 (T 3 ) and T 1 are only 0.22 (0.33) eV, and S 1 and T 2 states are nearly degenerate. It is well known that there is a strong vibrational coupling between two nearly degenerate excited electronic states. Therefore, the experimentally measured ISC rate of 7-PhQAD originates predominantly from the vibronic coupling effects, and the NACs between the low-lying triplet excited states play a key role. It is insufficient to reproduce such a fast experimental ISC rate if only the direct SOC between the first singlet and triplet state is incorporated because of the larger energy gap of 0.19 eV and the small SOCME of 0.14 cm -1 . At T=298 K, FC approximation produces a ISC/RISC rate for S 1 -T 2 crossing which is four orders of magnitude larger than that for S 1 -T 1 crossing. Considering the contribution of SVC to the rates, we conclude that the vibronic coupling significantly enhances the population transfer via the channel of S 1 -T 2 -T 1 .