A standard method to extract reaction coordinates from trajectories, either in biomolecular or reaction dynamics studies, is Principal Component Analysis (PCA) in cartesian coordinate space. However, this approach proved unsuccessful for the [Cu(C8PhenMe6)I] catalyst due to the difficulty in properly aligning this highly flexible system. The corresponding PCA does not show any separation between the three states (Figure ). To achieve good separation between the three states, educt, TS, and product, we developed a reduced internal coordinate description of the system (see Figure ) to minimize the noise from highly correlated coordinates. We used Bond, Angles, and Torsion coordinates and described rigid fragments, such as the individual calixarene units (cX), the phenanthroline bridge (Phena), the phenyl (Phe) and aniline (NPh) moieties by their respective centres of mass (Figure ). An overview over the distribution of the internal coordinates which define the reduced model can be found in the Supporting Information, Figure This internal coordinate set nicely separates educts and products in the PCA space (Figure ), but still shows overlap between educts and transition states. Analysing the loadings of the principal components (see SI Tables and), we can see that the main contributions belong to the coordinates defining the reaction centre (C-N, NPh-C, NPh-Phena distances), as well as to the distances between the product and the cage, describing changes in the coordination at the Cu centre as the product is formed and the settling of the product in the cavity.