Minimum gradient reaction path

The procedure and methods for the MEP determination by the NEB and parallel path optimizer methods have been explained in detail elsewhere [25, 27], Briefly, these methods are types of chain of states methods [20, 21, 25, 26, 30, 31]. In these methods the path is represented by a discrete number of images which are optimized to the MEP simultaneously. This parallel optimization is possible since any point on the MEP is a minimum in all directions except for the reaction coordinate, and thus the energy gradient for any point is parallel to the local tangent of the reaction path. 

Figure 2 shows a schematic representation of spin-allowed and spin-forbidden reactions. The total spin of the reaction is conserved in the spin-allowed path (So —> So or product 1 channel, which is the usual reaction path). This reaction path can be represented as a minimum energy (or valley) path connecting reactant and product. In between there exists a stationary point, that is, a point where the gradient of the potential energy with... 

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