Average path potential

In the chapter on reaction rates, it was pointed out that the perfect description of a reaction would be a statistical average of all possible paths rather than just the minimum energy path. Furthermore, femtosecond spectroscopy experiments show that molecules vibrate in many dilferent directions until an energetically accessible reaction path is found. In order to examine these ideas computationally, the entire potential energy surface (PES) or an approximation to it must be computed. A PES is either a table of data or an analytic function, which gives the energy for any location of the nuclei comprising a chemical system. 

Using a differential cost function such as that of Fiber and Karplus, the potential energy is averaged over the path by including a factor of 1/L. In other definitions, such as the one employed in the MaxFlux method, there is no such nonnalization. Therefore, if the potential is set to zero, the MaxFlux method will find that the best path is the straight line path connecting reactants and products. However, methods where the differential cost is proportional to 1 /L will find that all paths are equally good. 

Where A F(z) is the free energy at z relative to that at the reactant state minimum zr, and the ensemble average < > is obtained by a quantum mechanical effective potential [15]. Note that the inherent nature of quantum mechanics is at odds with a potential of mean force as a function of a finite reaction coordinate. Nevertheless, the reaction coordinate function z[r] can be evaluated from the path centroids r, first recognized by Feynman and Flibbs as the most classical-like variable in quantum statistical mechanics and later explored by many researchers [14, 15]. 

The Fourier coefficient average in the denominator of the last term is added to make the numerator and denominator symmetrical. It has no effect on the classical average. The classical factor AUa(x) signifies that the potential is evaluated at the centroid of the path... 

---