. Therefore, the HT-type vibronically induced term provides mixing of the triplet and singlet excited-states, coupled with the first order vibronic perturbation due to the nuclear displacement along with the 3N -6 normal modes Q. However, in this work, we account for the HTVC effect via directly evaluating the nuclear derivatives of SOC but not via calculating NAC vectors between the excited states. Considering large energy difference between S 1 and S n , and the small value