As demonstrated by the key geometrical parameters listed in Table MOs of S 1 and T 1 states, the SOCMEs of S 1 -T 1 are 0.14 and 0.15 cm -1 with respect to S 1 and T 1 geometries, respectively, which are relatively small. The experimentally measured ISC rate of S 1 -T 1 transition is quite large, however, suggesting that the contribution of vibronic coupling effects on the rates of S 1 -T 1 crossings should be significant. Here we thus calculate both the ISC and RISC rates with the inclusion of the FC, HTVC, and DR effects. A result comparison is made with the VG approximation. In our rate calculations, the experimental energy gap 1532 cm -1 is used for S 1 -T 1 transitions, whereas for S 1 -T 2 crossings, we set ∆E S 1 T 2 = 74 cm -1 as the calculated adiabatic energy gap between S 1 and T 2 by TDA-CAM-B3LYP*. Table shows both the calculated and experimentally measured ISC and RISC rates for S 1 -T 1 and S 1 -T 2 crossings. For S 1 -T 1 crossings, the calculated ISC rates are in the same order of magnitude at three different temperatures (50, 100, and 298 K), indicating that the temperature effect on this ISC process is not so evident in the range of 50-298 K. On the other hand, the HT effect increases the ISC rate by more than one order of magnitude. For instance, at T=298 K and without VG approximation, the FC-and HT-like contributions in ISC rates are 3.4 × 10 7 and 5.4 × 10 8 s -1 , respectively. The calculated results with the VG approximation show the similar tendency. The deviations between the calculated values with and without VG approximation are small, indicating that there is no much large difference between the PESs of two concerned singlet and triplet states. Regardless of whether VG approximation is invoked or not, the contribution of HTVC is much larger than FC. It is thus no doubt that the experimental ISC rate originates predominately from the contribution of the vibronic coupling.