Force field refinement for reproducing experimental infrared spectra of ionic liquids.

We employ polarizable molecular dynamics simulations with the newly developed FFGenOpt parametrization tool to reproduce IR spectra of several ionic liquid cations and anions in the gas phase. Our results show that polarizable force fields in the bulk phase provide a reasonable compromise between computational effort and accuracy for investigating IR spectra when treating the transition from gas to liquid phase carefully. Although collectivity seems to play only a minor role, the liquid phase not only changes the electrostatic environment of the molecules but also introduces friction and intermolecular interactions altering the IR spectrum significantly. In addition to the classical force field approach, we also tested if the additional computational effort of machine learning potentials justifies their application in reproducing IR spectra. However, the main purpose of this work is to improve the quality of polarizable force fields concerning vibrations and not the prediction of IR spectra which can be better done with quantum-mechanical cluster approaches.