Does Explicit Polarizability Improve Molecular Dynamics Predictions of Glass Transition Temperatures of Ionic Liquids?

Molecular-dynamics simulations are used for predictions of the glass transition temperatures for a test set of 5 aprotic ionic liquids. Glass transitions are localized with the trend-shift method analyzing volumetric and transport properties of bulk amorphous phases. Classical non-polarizable all-atom OPLS force-field model developed by Canongia Lopes and Pádua (CL&P) is employed as the starting level of theory for all calculations. Alternative approaches of charge scaling and Drude oscillator model, accounting for atomic polarizability either implicitly or explicitly, respectively, are used to investigate the sensitivity of the glass transition temperatures to induction effects. The former non-polarizable model overestimates the glass transition temperature by tens of Kelvins (37 K in average). The charge scaling technique yields a significant improvement, and the best estimations were achieved using polarizable simulations with the Drude model, which yielded an average deviation of 11 K. Although the volumetric data usually exhibit a lesser trend shift upon vitrification, their lower statistical uncertainty enables to predict the glass transition temperature with a lower uncertainty than the ionic self-diffusivities, the temperature dependence of which is usually more scattered. Additional analyses of the simulated data were also performed, revealing that the Drude model predicts lower densities for most sub-cooled liquids, but higher densities for the glasses than the original CL&P, and that the Drude model also invokes some longer-range organization of the sub-cooled liquid, greatly impacting the temperature trend of ionic self-diffusivities in the low-temperature region.