In QM methods, molecules are more accurately described than in MM through the usage of atomic and molecular orbitals to describe atoms in molecules (Box 1). In terms of computational cost and time requirements, QM calculations are orders of magnitude more costly than MM. However, the major advantages of QM over MM when it comes to predicting TS structures are 1) the ability to optimize structures without the need for specific parameters (assuming all the necessary elements -in particular transition metals -are included in the basis set, see Box 1) and 2) obtaining accurate energies and geometries that describe the bond breaking/forming process. While some QM methods (i.e., ab initio: