Multi-state vibronic dynamics

Finally, in Chap. 5 which closes Part I, an application of the tools introduced in Chaps. 2 and 4 to a quantum dynamical investigation of the photophysics of pyrazine is presented. This work focuses on the role of the low-lying dark wtt states in the non-adiabatic dynamics of the molecule after photoexcitation. Multi-reference electronic structure calculations are used to design a vibronic couping model Hamiltonian, including the four lowest electronic states and the sixteen most important vibrational modes. This model is then used to simulate the absorption spectrum and the ultrafast decay dynamics of the molecule using the MCTDH method. 

In the previous sections, the Born-Oppenheimer approximation was assumed, i.e. nuclear motion on a single PES was considered. However, in many simations, the dynamics of the nuclei need to be treated on several PESs corresponding to coupled electronic state. The coupling between the electronic states can be due to the presence of an external electric field or to internal vibronic interactions. There exists two different ways of treating several coupled electronic states with the MCTDH method [60], the single-set formulation and the multi-set formulation. 

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