Potential energy surfaces bifurcations, transition states

Whereas the equilibrium structure itself is pretty much agreed on for the water dimer, there are a number of other geometries that one might suppose should be comparable in energy. For example, the notion of a cyclic structure has been advanced over the years as have various types of bifurcated geometries. Smith et al. recently completed the most comprehensive survey of the ab initio potential energy surface of the water dimer to date. They examined the question of a number of possible minima on the surface and potential transition states that connect them. 

Figure 8.3 A schematic two-dimensional view of the potential energy surface and wave-packet dynamics in the ultrafast photodissociation of Hgl2 [adapted from Voth and Hochstrasser (1996), Zewail (1996)]. The transition state for the I + Hgl reaction is along the bisector, dashed line, with the lowest barrier at the bottom of the potential along that line. The UV excitation creates a localized wave-packet along the bisector. The center of the packet is displaced from the saddle point to a compressed configuration along the symmetric stretch. During the dissociation the wave-packet bifurcates, as shown, and each component is followed in the figure. It shows the coherent vibrational motion in the Hg—I well. ...

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