Investigation of the spin-forbidden process in thymine

The intersystem crossing rate from the lowest singlet to the lowest triplet state of thymine was studied by means of the ab initio methods. The rate was calculated employing the time-dependent approach based on the correlation function. The normal modes of the singlet and triplet electronic states are related by the Duschinsky transformation, i. e. by rotation and translation. The correlation function was calculated using the Condon approximation for the spin-orbit matrix element and harmonic approximation for the nuclear motion. The intersystem crossing rate strongly depends on the singlet-triplet adiabatic energy gap and on the normal mode mixing.