Here, to demonstrate the effectiveness of the new approach, we consider target IV, the monomeric repeating unit of a well-known polymer, namely polyethylene terephthalate (PET), which consists of a benzene ring covalently bonded to a carboxyl group and an ethylene glycol moiety (see Figures 7.a and S12 in the Supporting Information). As shown in the b) to d) panels of Figure , when compared to the QM reference (see Table ) an accurate representation of the monomeric unit conformational features is not achievable by simply transferring parameters from general-purpose FFs (see also Figures and in the Supporting Information). Yet, a reliable description of the internal flexibility is crucial to PET's properties and functions. It is thus evident that, in view of extending the model to longer polymeric chains, the intramolecular FF describing each unit should be accurately refined.