Abstract
Ionic liquids are an interesting class of soft matter with viscosities of one or two orders of magnitude higher than that of water. Unfortunately, classical, non-polarizable molecular dynamics (MD) simulations of ionic liquids result in too slow dynamics and demonstrate the need for explicit inclusion of polarizability. The inclusion of polarizability, here via the Drude oscillator model, requires amendments to the employed thermostat, where we consider a dual Nosé-Hoover thermostat, as well as a dual Langevin thermostat. We investigate the effects of the choice of a thermostat and the underlying parameters such as the masses and force constants of the Drude particles on static and dynamic properties of ionic liquids. Here, we show that Langevin thermostats are not suitable for investigating the dynamics of ionic liquids. Since polarizable MD simulations are associated with high computational costs, we employed a self-developed graphics processing unit enhanced code within the MD program CHARMM to keep the overall computational effort reasonable.
Highlights
Aqueous concentrated salt solutions and ionic liquids, i.e., salts which are liquid at ambient temperatures, have numerous applications in chemistry, physics, and energy storage/conversion.1,2 Often molecular dynamics (MD) simulations are employed to gain fundamental insights into their structural and transport properties
Classical, non-polarizable molecular dynamics (MD) simulations of ionic liquids result in too slow dynamics and demonstrate the need for explicit inclusion of polarizability
We investigate the effects of the choice of a thermostat and the underlying parameters such as the masses and force constants of the Drude particles on static and dynamic properties of ionic liquids
Summary
Aqueous concentrated salt solutions and ionic liquids, i.e., salts which are liquid at ambient temperatures, have numerous applications in chemistry, physics, and energy storage/conversion. Often molecular dynamics (MD) simulations are employed to gain fundamental insights into their structural and transport properties. Often molecular dynamics (MD) simulations are employed to gain fundamental insights into their structural and transport properties. The prediction of physico-chemical properties of such systems employing MD simulations often requires explicit inclusion of polarizable forces to damp the Coulomb interactions between the ions.. Non-polarizable MD simulations are able to reproduce structural experimental data, they usually result in dynamics which are almost one order of magnitude too slow compared to the experiment.. Polarizable ionic liquid simulations were shown to produce dielectric spectra, conductivities, diffusion coefficients, viscosities, and solvation dynamics in close agreement with the experiment. Despite their high accuracy, the increased computational effort of polarizable simulations often limits their use. Due to the very small self-diffusion constants of ionic liquids, realistic diffusive behavior can only be observed by simulating several dozens to hundreds of nanoseconds. The need for long simulation times, together with the slow implementation of polarizability in common MD packages, renders polarizable simulations of ionic liquids difficult and time-consuming
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