fixing the relevant bond length in the pathway that leads to the final product, while all other degrees of freedom undergo geometry optimization in the so-called constrained scheme ; ii) by providing a guessed TS structure alongside the initial/final points of the reaction pathway for the QST3 optimization or iii) by optimizing a guessed TS structure to a first-order saddle-point, using the Berny algorithm and following the eigenvector with most negative eigenvalue. In either case, the saddle point is confirmed by the presence of one negative frequency along the reaction path.