Reaction path

The reaction path will connect the several chemical species that evolve during the reaction process. The word "connected" needs to be clarified. The most adequate definition of such connection is the trajectories that an ensemble of molecular entities travel through the PES at a given temperature until the reaction is completed. This is the most general definition of reaction paths and must be studied by statistical mechanics [137]. However, it can be studied in a more simplified definition by optimization methods[138,139].

Optimization methods provide very valuable information and may give a chemical insight where statistical methods are far from being applicable. Therefore, restricting our study to one molecule, one stationary point and one valley of reaction without the effect of temperature, we can associate the reaction path as the path by which stationary points are connected through the lowest energy valley in the PES. This path is called the minimum energy path (MEP).

In some cases, such as small and/or gas phase systems, the MEP is a very good approximation to the reaction paths. Strictly, this MEP would be, without considering the zero point energy correction, the reaction path that would follow an hypothetical system at zero Kelvin.

In this section some methods to find the MEP are commented. In addition, some of these methods are useful to find a geometry guess to start a saddle point free-search.