Chemical direct dynamics trajectory mechanism

The remainder of this chapter is organized as follows. Born-Oppen-heimer direct dynamics is reviewed in the next (second major) section. Approaches for integrating the classical equations of motion are outlined in the third major section (on integrating classical equations of motion). Algorithms for choosing initial conditions for ensembles of trajectories representing unimolecular and bimolecular reactions are reviewed in the fourth major section. The adequacy of classical mechanics for describing the dynamics of chemical processes, and the possible importance of quantum effects, is reviewed in the fifth major section. The final section surveys several applications of direct dynamics. 

The modified spectator stripping model (polarization model) thus appears to be a satisfactory one which explains the experimental velocity distribution from very low to moderately high energies. The model emphasizes that the long-range polarization force has the dominant effect on the dynamics of some ion—molecule reactions. However, a quite different direct mechanism based on short-range chemical forces has been shown to explain the experimental results equally satisfactorily [107, 108]. This model is named direct interaction with product repulsion model (DIPR model) and was originally introduced by Kuntz et al. [109] in the classical mechanical trajectory study of the neutral reaction of the type... 

Three novel approaches to the simulation of NA dynamics of large chemical systems have been presented [20-22]. The approaches extend the standard quantum-classical NA MD to incorporate quantum effects of the solvent subsystem that have been traditionally treated by classical mechanics. These effects include quantum trajectory branching (wave packet splitting), loss of quantum coherence directly related to the Franck-Condon overlap contribution to the NA transition probability, and ZPE of nuclear motion that contributes to the NA coupling and must be preserved during the equilibration of the energy released by the NA transition. 

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