The relative energetic stability of the “rac” and the “meso” rotational isomers of metallocene-based Ziegler−Natta (ZN) catalysts plays an important role in determining the structural, physical and chemical properties of synthesized polymers. Due to the large molecular size of these systems, ab initio calculations are often prohibitive. To circumvent this problem, we use the QM-Pot approach, which treats the metal center and its surrounding atoms/ligands by quantum mechanics (QM), whereas the (organic) atoms/functional groups away from the metal center are described by an interatomic potential function (Pot). As a concrete example, we choose a commercially important zirconocene-based catalyst and compute: (1) the relative energies of the rac and meso structures; (2) the activation barrier for the rotational transition from one to the other; and (3) the effect of the addition of organic functional groups on the above energies. Due to small mismatches between QM and Pot potential surfaces, one has to be ca...