表面跳跃法模拟DNA碱基胸腺嘧啶的运动-系间穿越
The DNA Nucleobase Thymine in Motion - Intersystem Crossing Simulated with Surface Hopping
摘要：
基于CASSCF势能面和\textsc{Sharc}表面跳跃方法，对孤立胸腺嘧啶进行了从头算激发态动力学模拟，研究了系统间跃迁的机制.我们表明，即使$S_2 \rightarrow S_1$内部转换没有准确地描述与CASSCF，系统间的交叉可以正确地模拟。胸腺嘧啶的系统间交叉发生在$S_1$（$^1nπ^*$）最小值处，通过附近与$T_2$（$^3ππ^*$）的交叉。该系统通过三重态流形中的超快内转换进一步弛豫到$T_1$（$^3ππ^*$）态。模拟结果表明，一旦系统陷入$^1nπ^*$极小值，系统间的穿越将以1 ps的时间常数进行.此外，系统的电子状态的变化分别伴随着特定键的伸长/缩短，其因此可以用作标识在动态中填充哪个状态的指示器。
英文摘要：
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 \textsc{Sharc} surface hopping method. We show that even though $S_2 \rightarrow S_1$ internal conversion is not described accurately with CASSCF, intersystem crossing can be correctly simulated. Intersystem crossing in thymine occurs from the $S_1$ ($^1nπ^*$) minimum, via a nearby crossing with $T_2$ ($^3ππ^*$). The system further relaxes via ultrafast internal conversion in the triplet manifold to the $T_1$ ($^3ππ^*$) state. The simulations reveal that, once the system is trapped in the $^1nπ^*$ 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.
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