Finally, we derived new dihedral parameters for torsions that had not previously been optimized for the Drude force field. Careful tuning was required to guarantee that our model would capture the conformational dynamics of PET polymer and its derived molecules. As shown in Figure and 3B, we used 2HEB to derive parameters for the bond between the ester carbon and the aromatic ring and the ester rotatable bond (defined in Figures and, respectively). For both torsions, our Drude parameters were able to reproduce the energy minima and barrier heights of ∼6 kcal/mol and ∼11 kcal/mol. For torsions associated to the ethylene glycol (EG) linkage between PET residues, we used EGDA to scan the dihedrals CX2-O3-CD-CA and O3-CD-CA-O1 as defined in Figure and S3D and shown in Figure and 3D, respectively. The torsion O3-CD-CA-O1 is commonly defined as Ψ and is directly tied to the gauche (Ψ g ∼ 70°) and trans (Ψ t ∼ 180°) torsional populations in crystalline or amorphous PET. In our torsional profiles, two equivalent minima are located at the gauche angles with the trans configurations ∼0.85 kcal/mol higher, in line with relative energetics proposed by Schmidt-Rohr et al.