Developing accurate intramolecular force fields for conjugated systems through explicit coupling terms

The accuracy of molecular mechanics force fields (FF) reveals critical for applications where precise molecular structures along a conformational sampling are required, as in the simulation of electronic spectroscopies. This implies abandoning generalized FFs in favor of specific FFs, with non-transferable parameters able to accurately describe the targeted species. A promising strategy in this direction consists in the so-called quantum mechanically derived FFs, in which the parameters are fitted onto reference data computed through quantum chemistry. However, in order to obtain a global set of parameters able to reliably describe the reference potential energy surface in different regions of the conformational space, the complexity of the analytical expressions of the FF becomes crucial. Regarding intramolecular interactions, the functional form of standard transferable FFs is restricted to terms that depend on only one internal coordinate. It will be shown that such models may reveal insufficient to describe systems as polyenic chains, where complex electronic effects, e.g., conjugation, intrinsically couple different internal coordinates (ICs). We propose a functional form for intramolecular FFs, which includes explicit couplings between flexible dihedrals and stiff ICs (bonds and angles), being able to properly describe the geometrical changes arising not only from steric interactions, but also from conjugation effects, i.e., the change of bond orders induced by conformational changes. The parameterization of the coupled FFs is carried out by means of automated and efficient computational protocols, purposely developed in the present work. All procedures are tested and validated by generating FFs for the two smallest compounds in the polyenic series (butadiene and hexatriene).