Nonadiabatic effects diabatic states

Let us summarize this chapter. Nonadiabatic effects in photodissociation are very important for many molecular systems. The higher the excitation energies the more likely the necessity of considering more than one electronic states becomes. The propagation of wavepackets in two or even several electronic states is rather straightforward today in comparison with the construction of the relevant adiabatic or diabatic potentials and the coupling between them. 

We compare state-to-state reaction probabilities and ICS obtained by three sets of calculations, two of them using the ground electronic state PES in which the GP is either included (IS-GP) or excluded (IS-NGP), and the last one includes all of the nonadiabatic effects using the two coupled diabatic surfaces (2S-DIABATIC). 

For arbitrary triangles (the Q point group), the angle 6 is not uniquely defined, but can be obtained by an analysis of the wave function in a way similar to that used to construct the diabatic states from the adiabatic ones [180,181]. It should also be noted that for arbitrary trimer configurations, the resulting two A states are coupled nonadiabatically. This coupling can be viewed as a pure three-body effect. 

The first three terms on the right-hand side correspond to the Marcus relation for the nonadiabatic case where there is no coupling between the diabatic energy states (i.e. E = 0 at all values of the reaction coordinate). The fourth and fifth terms reflect the effect of the adiabatic coupling of the two surfaces on the transition state and reactant state, respectively, and < /2 AGjxn + 4AGin,l. ... 

While, as discussed above, general solutions to the multistate nonadiabatic problem in Eq. 8.6 are out of reach for large systems, the latter can be suitably tackled for semirigid systems due to MVCM models based on a quasi-diabatic description of the electronic states. Such models allow for effective computations when the harmonic approximation is, at least, a good starting point for the description of the diabatic... 

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