The DNA nucleobase thymine in motion - Intersystem crossing simulated with surface hopping

We report ab initio excited-state dynamics simulations on isolated thymine to investigate the mechanism of intersystem crossing, based on CASSCF potential energy surfaces and the SHARC surface hopping method. We show that even though S2 -> S1 internal conversion is not described accurately with CASSCF, intersystem crossing can be correctly simulated. Intersystem crossing in thymine occurs from the S-1((1)n pi*) minimum, via a nearby crossing with T-2 ((3)pi pi*). The system further relaxes via ultrafast internal conversion in the triplet manifold to the T-1((3)pi pi*) state. The simulations reveal that, once the system is trapped in the (1)n pi* minimum, intersystem crossing might proceed with a time constant of 1 ps. Furthermore, the change of the system's electronic state is accompanied respectively by elongation/shortening of specific bonds, which could thus be used as indicators to identify which state is populated in the dynamics. (C) 2016 Elsevier B.V. All rights reserved