Potential energy surfaces limitations

Most of the reaction path approaches make use of a spatial step and accordingly are not affected by the time-scale limitation of other MD methods. For complex systems, however, the ruggedness of the potential energy surface limits the applicability and accuracy of these paths because the number of conformations in the trajectory needs to increase. Another fundamental limitation is that these methods are not applicable for the study of molecular events for which very few details are known about the conformations of products or key intermediates. It is for those processes where theoretical approaches are more helpful. 

Figure 3. Relaxed triangular plot [68] of the U3 ground-state potential energy surface using hyperspherical coordinates. Contours, are given by the expression (eV) — —0.56 -t- 0.045(n — 1) with n = 1,2,..,, where the dashed line indicates the level —0.565 eV. The dissociation limit indicated by the dense contouring implies Li2 X Sg ) -t- Li.

Before discussing the kinds of kinetic information provided by potential energy surfaces we will briefly consider methods for calculating these surfaces, without going into detail, for theoretical calculations are outside the scope of this treatment. Detailed procedures are given by Eyring et ah There are three approaches to the problem. The most basic one is purely theoretical, in the sense that it uses only fundamental physical quantities, such as electronic charge. The next level is the semiempirical approach, which introduces experimental data into the calculations in a limited way. The third approach, the empirical one, makes extensive use of experimental results. 

The second example is the quadratically chirped pump-dump scheme. Since the pioneering work by Tannor and Rice [119], the pump-dump method has been widely used to control various processes. However, since it is not possible to transfer a wave packet from one potential energy surface to another nearly completely by using the ordinary transform limited or linear chirped pulses, the... 

Fig. 3. Projections on the (<1>, maps of the CICADA conformational search of the pentasaccharide. The dots indicate the values of all the optimized conformations determined by CICADA at each glycosidic linkange in 8 kcal/mol energy window For comparison, the isocontours, drawn in 1 Kcal/mol steps with an outer limit of 8 kcal/mol, represent the energy level of each disaccharide and calculated with the relaxed grid search approach. Dashed regions represent the locations of the low energy conformation of the pentasaccharide plotted on the potential energy surfaces of the constituting disaccharide segments...

However, it is not known into which part of the potential energy surface these species couple. The reactions of H2CO+ with OH and CH2OH+ with O-atoms would also access the surface although these are not experimentally very tractable. The surface is also accessed to a limited extent by the gas kinetic proton transfer from HCO+ to H20 yielding H30+. 

Figure 5. Potential energy surface for the C2HsO+ system, showing stable structures, transition states and dissociation limits. The information is predominantly from the theoretical calculations of Fairley et al.71 with the region of the HCO+ + CH4 and CH3 + H2CO dissociation limits from Audier et al.75...

The potential energy surface (PES) Up(Qk) for the excited electron state p has its minimum near the point Q (Fig. 1). In the classical limit, the electron transition may be treated as a continuous motion of the system on the lower PES, Ua, from the... 

---