Quasi-Classical Trajectory Studies Of Polyatomic Molecular Reaction Dynamics

Molecular reaction dynamics is an important subject to study the microscopic mechanism of chemical reactions.Potential energy surface(PES)is the cornerstone of molecular reaction dynamics.The dynamic quantities,such as reaction rate,integral cross section(ICS),differential cross section(DCS),the distribution of the vibrational-rotational of the products can be obtained based on the PES.Nowadays,quantum dynamics(QD)and quasi-classical trajectory(QCT)dynamics have achieved great success in triatomic and tetra-atomic systems.However,for the systems involving more than four atoms,due to the increase of the degree of freedom,the amount of computation increases dramatically and QD calculations become difficult.In this work,the dynamics of polyatomic molecular reactions is studied using the QCT method on existing or newly constructed PES.In the first chapter,the background of molecular reaction dynamics is mainly introduced.The second chapter presents the concept,construction method of PES and the basic theory of QCT.The main work is given in chapters 3,4,5 and 6.The third chapter introduces the theory and new scheme of final state analysis.In the fourth chapter,we studied the effects of the vibration excitation of the reactant HBr,the symmetric stretching and the umbrella excitation of CH3 on the reactivity of the HBr +CH3 → Br + CH4 reaction and analyzed why the vibrational adiabatic model breaks down.In the fifth chapter,we use the newly constructed PES of the OH + H2O → H2O+ OH reaction to investigate the mode-specific state-to-state dynamics.The energy flow in the reaction process is unveiled.It is found that OH only acts as a spectator during the reaction.The symmetric stretching and anti-symmetric stretching mode of the reactant H2O play a significant role in the reaction.The promotional effect of the bending mode on reaction is weaker than the stretching mode.In the sixth chapter,we constructed an accurate global PES for the ground electronic state of the OH + H2S-H2S + SH reaction based on～80000 ab initio points at the level of UCCSD(T)-F12a/aug-cc-pVTZ.The PES is fitted using the fundamental invariant-neural network(FI-NN)method with a root mean squared error of 4.72meV.The reaction dynamics is then studied by the QCT method on this PES.In this reaction,the ICS increases with the translational energy,and the DCS has a pronounced backward and sideway scattering distribution.The results show that the symmetric stretching and anti-symmetric stretching modes of H2S have the same promotional effect on the reaction.