光化学反应是自然界中非常重要的反应，与生命的繁衍息息相关。由于分子在激发态的势能面非常复杂，有着非常多的过渡态，极小点，避免交叉和锥形交叉等等，对于理论科学家来说，研究光化学反应一直是一项非常具有挑战性的工作。电子结构计算可以给出很多信息来推测光化学反应的机理，比如极小点（minimum），过渡态（TS），最低能量交叉点（MECI），最低能量反应路径（MEP）等，但是，大多数的光化学反应都是在飞秒尺度上发生的，也就是说，整个反应过程是处于非平衡的状态。因此，直接采用动力学模拟的方法来研究光化学反应机理是最为明智的选择。本文主要从电子结构计算和动力学模拟两个方面来阐述一些有机小分子发生光化学反应的机制，本论文的主要研究内容包括：（1）光诱导的乙烯基桥联偶氮苯（BA）顺式（1Z）和反式（1E）构型间的异构化反应机制。作者采用不同等级的从头算电子结构计算方法得到BA在S0和S1态的平衡点以及二维势能面，同时采用Ehrenfest非绝热动力学模拟1E→1Z和1Z→1E的光异构化过程。计算结果与实验一致，BA顺式比反式构型更加稳定，而自由旋转的偶氮苯反式构型具有更高的稳定性。从S0态的二维势能面可以看出BA的热异构化是通过平面内的转动（inverse）实现的，在发生异构化的过程中需要翻越一个很高的势垒。而1E→1Z和1Z→1E的光异构化过程都是经过S1和S0态的交叉点实现的，并且以脚踏板的机制发生。对于1E构型，其S1态的寿命只有~30 fs；而对于1Z构型，S1态的寿命要远远大于这个时间尺度，这是由于物质受光激发到S1态后能量的重新分布过程不一样导致的，或者是Ehrenfest方法本身，抑或是激光脉冲的激发过程影响导致。BA的2D势能面和在S1态上的动力学过程很好地解释了1E→1Z比1Z→1E光异构化效率更高的原因。当分子经过交叉点跃迁回基态之后，1E的形成是一个无势垒的过程，而1Z的形成则需要翻越一个小的势垒，这与实验中1Z→1E具有更高的量子产率是一致的。（2）反式质子化席夫碱3（PSB3）的光异构化过程的机理。作者采用CASSCF和MSPT2水平上的电子结构计算以及CASSCF//AIMS和MSPT2//AIMS的动力学模拟研究了反式质子化席夫碱3（PSB3）的光异构化过程。无论是平衡点的电子结构计算，还是动力学模拟，CASSCF和MSPT2都给出了不同的结果。这就意味着对于PSB3这一体系，电子动态相关效应非常明显。另外，尽管电子结构计算可以给出电子态势能面的一些信息，比如极小点、过渡态、最低能量交叉点、甚至连接两个最低能量交叉点的最低能量交叉束路径（minimum energy conical intersection seam paths），但是在实际的动力学模拟过程中，所有发生非绝热跃迁附近的交叉点都在能量上远离最低能量交叉点和连接最低能量交叉点的最低能量交叉束路径，这是由于实际的动力学过程非常快、处于高度不平衡的状态。因此，如果想要真正地了解光化学反应的机理，采用高精度的从头算动力学模拟是非常有必要的。（3）环丙烯酮（CP）的光解离机制。作者采用了在CASSCF和MSPT2水平上的电子结构计算和CASSCF//AIMS的动力学模拟研究了环丙烯酮（CP）的光解离机制，MSPT2的电子结构计算发现动态电子相关效应对本体系的反应机制影响甚微。通过电子结构计算，在S0和S1态上存在极小点和两个最低能量交叉点，并且计算得到了最低能量交叉点的势能面剖析图特征。通过对CP激发到S1态的非绝热动力学模拟发现，S1态的寿命为125 fs（采用单指数函数进行拟合），仅有少量的CP通过CI-2的交叉点回到基态，这与最低能量交叉点CI-2具有相对更高的能量和相对Slope的拓扑结构特征是一致的。处于S1态的CP主要是通过分步反应的机制完成光解离过程，首先通过类似CI-1的交叉点附近（其中一个C-C单键断裂）回到基态，之后在基态上完成解离生成CO和乙炔的基态产物（另一个C-C单键断裂）。当然，CP还有很小的几率通过类似CI-2的交叉点附近（仅C=C双键的拉伸）回到基态，然后按照协同非同步的机理完成解离过程。最后，通过研究过剩能量（E1-ECI-1；Eexcess）、S1和S0态能量差（Egap）与跃迁效率的关系发现，超快的光化学反应是在高度不平衡的情况下发生的，采用从头算的动力学模拟去研究光化学反应的机制是非常有必要的。

Photo-induced chemical reactions, as a kind of very important reaction in nature, have fascinated theoretical and experimental chemists for decades. It is a real challenge to do the theoretical study on photochemical process, because the potential energy surfaces (PESs) of excited states could be very complicated, exhibiting multiple avoid crossing and conical intersection. Electronic structure calculations could provide some information about the mechanism of chemical reaction, such as minima, transition states, minimal energy conical intersections (MECIs), and minimal energy path (MEP). However, many photochemical processes take place on the femtosecond timescale in nonequilibrium conditions, which is difficult to be described accurately by the methods above. Direct dynamics simulation is the best choice to obtain a complete picture of the mechanistic photochemistry for polyatomic molecule. This reseach is organized as follows: (1) Mechanism of phototriggered isomerization of ethylene bridged azobenzene (BA). The S0 and S1 potential energy surfaces of the ethylene-bridged azobenzene that was recently reported to have highly efficient photoisomerization were determined by ab initio electronic structure calculations at different levels and further investigated by a semiclassical dynamics simulation. Unlike azobenzene, the cis isomer of BA was found to be more stable than the trans isomer, consistent with the experimental observation. The thermal isomerization between cis and trans isomers proceeds via an inversion mechanism with a high barrier. Interestingly, only one minimum-energy conical intersection was determined between the S0 and S1 states (CI) for both cis → trans and trans → cis photoisomerization processes and confirmed to act as the S1 → S0 decay funnel. The S1 state lifetime is ∼30 fs for the trans isomer, while that for the cis isomer is much longer, due to a redistribution of the initial excitation energies. The S1 relaxation dynamics investigated here provides a good account for the higher efficiency observed experimentally for the transfcis photoisomerization than the reverse process. Once the system decays to the S0 state via CI, formation of the trans product occurs as the downhill motion on the S0 surface, while formation of the cis isomer needs to overcome small barriers on the pathways of the azo-moiety isomerization and rotation of the phenyl ring. These features support the larger experimental quantum yield for the cis → trans photoisomerization than the trans → cis process.(2) Ab initio multiple spawning simulation of trans-Pronated Schiff Base 3 in gas phase by the method of MSPT2 with analytical derivative coupling, focus on the dynamic correlation effect on photochemical reaction of PSB3. Both static electronic structure calculations and dynamics simulations show different results between the CASSCF and MSPT2 levels. Dynamic electronic correlation is shown to be critical in the photo-isomerization reaction.While the static electronic structure calculations can give a landscape of the electronic potential energy surfaces, the photo-dynamicsof trans-PSB3is dominated by neither minimum energy conical intersections (MECIs) nor minimum energy conical intersection (CI) seampaths. Most CIs involved in the photo-reaction process are far away from MECIs and minimum energy CI seam paths, which demonstrate the excited state dynamics does not follow the minimum energy path at all.(3) The pholysis mechanism of cyclopropenone. Equilibrium structures of cyclopropenone in the S0 and S1 states and the relative energies of the low-lying excited singlet states have been determined by the SA4-CAS(10,9) and MS-SA4-CASPT2(10,9) calculations with different basis sets. Two minimum-energy conical intersections (CI-1 and CI-2) between S0 and S1 were determined and their topographic characters were characterized by the SA4-CAS(10,9) calculated energy gradients and non-adiabatic coupling vectors. The AIMS method was used to carry out non-adiabatic dynamics simulation with ab initio calculation performed at the SA4-CAS(10,9) level. Based on time evolution of wave functions simulated here, the S1 lifetime is fitted to be 125 fs with a pure exponential decay for the S1 population. The CI-1 intersection is mainly responsible for ultrafast S1→S0 non-adiabatic transition and the photo-induced decarbonylation is a sequential process, where the first C-C bond is broken in the S1 state and fission of the second C-C bond occurs in the S0 state as a result of the S1→S0 internal conversion via the CI-1 region. As a minor channel through the CI-2 region, the decarbonylation proceeds in an asynchronous concerted way. The influence of the S1 excess energies (Eexcess) and the S1-S0 energy gap (Egap) on the S1→S0 transition via the CI-1 region has been explored by a change of the S1 population with Eexcess and Egap. It was found that the S1→S0 non-adiabatic transition occurs within a small energy gap and the transition probability exhibits an approximate Gaussian distribution in view of the S1 excess energies. The present study provides new insights into mechanistic photochemistry of cyclopropenones and reveals that the AIMS dynamics simulation at a high-accuracy ab initio level is a powerful tool for investigating the mechanism of an ultrafast photochemical reaction.