Reaction paths, 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. 

Then, in the temperatiare range T>T- /2 the empirical Polanyi-Semenov relations (224.Ill) become identical with the theoretical equations (63.1) based on the properties of the potential energy surfaces. However, the condition T>T /2 is necessary for the validity of equations (231.Ill) but is not sufficient for the realization of inequalities (240.Ill) which lead to equations (241.III). The latter require the stronger limitation so that the reaction proceeds entirely in the classical temperature range T>2T in which the nuclear tunneling is quite negligible. This requirement may be sufficient if the motion along the classical reaction path is very fast compared to the nonreactive modes. Then, expression (216.Ill) yields, indeed, E, since for 8 =... 

The kineties of eleetron-transfer reactions, which is also affected by the electrode potential and the metal-water interface, is more difficult and complex to treat than the thermodynamic aspects. While the theoretical development for electron transfer kinetics began decades ago, a practical implementation for surface reactions is still unavailable. Popular transition state-searching techniques such as the NEB method are not designed to search for minimum-energy reaction paths subject to a constant potential. Approximations that allow affordable quantum chemistry calculations to get around this limitation have been proposed, ranging from the electron affinity/ionization potential matching method to heuristic arguments based on interpolations. 

---