Intrinsic reaction coordinate

It is important to note that while the stationary points are invariant to any coordinate transformation, the MEP is not. It means that it will depend, to some extent, on the coordinate system. The MEP located in mass-weighted coordinates is unique and it is called the Intrinsic Reaction Coordinate (IRC) [141] ^2.7. There are several methods that follow the IRC from the saddle point down to the reactants and down to the products [139]. All of them try to solve the steepest descent reaction path as defined in the following differential equation

$$\frac{ d {\bf x}(s) }{ d s} = - \frac{{\bf g}(s)}{\vert{\bf g}(s)\vert}$$ (2.90)