Resolving dispersion and induction components for polarisable molecular simulations of ionic liquids

Abstract

One important development in interaction potential models, or atomistic force fields, for molecular simulation is the inclusion of explicit polarisation, which represents the induction effects of charged or polar molecules on polarisable electron clouds. Polarisation can be included through fluctuating charges, induced multipoles or Drude dipoles. This work uses Drude dipoles and is focused on room-temperature ionic liquids, for which fixed-charge models predict too slow dynamics. The aim of this study is to devise a strategy to adapt existing non-polarisable force fields upon addition of polarisation, because induction was already contained to an extent, implicitly, due to parametrisation against empirical data. Therefore, a fraction of the van der Waals interaction energy should be subtracted so that the Lennard-Jones terms only account for dispersion and the Drude dipoles for induction. Symmetry-adapted perturbation theory (SAPT) is used to resolve the dispersion and induction terms in dimers and to calculate scaling factors to reduce the Lennard-Jones terms from the non-polarisable model. Simply adding Drude dipoles to an existing fixed-charge model already improves the prediction of transport properties, increasing diffusion coefficients and lowering the viscosity. Scaling down the Lennard-Jones terms leads to still faster dynamics and to densities that match experiment extremely well. The concept developed here improves the overall prediction of density and transport properties and can be adapted to other models and systems. In terms of microscopic structure of the ionic liquids, the inclusion of polarisation and the down-scaling of Lennard-Jones terms affects onyl slightly the ordering of the first shell of counterions, leading to small decreases in coordination numbers. Remarkably, the effect of polarisation is major beyond first neighbours, significantly weakening spatial correlations, a structural effect that is certainly related to the faster dynamics of polarisable models.

Molecular Dynamics Simulations of Ionic Liquids and Electrolytes Using Polarizable Force Fields

“…For the design of a polarizable force field, several strategies have evolved:All Lennard-Jones parameters are reparametrized after adding the polarizable forces to the simulation.The ratio between the interaction of the induced dipoles and dispersion can be determined by DFT calculations using symmetry-adapted perturbation theory. 247 As shown in Table 3, dispersion entirely dominates the interaction between hydrocarbons, 248 whereas the contribution of the induced dipoles become almost an equal partner in case of the interaction between the ions. The two values for the NTf 2 -anion discriminate between the oxygens or the fluorine of the NTf 2 being closest to the hydrocarbon.The Lennard-Jones parameter are not reparametrized individually but scaled according to the polarizability of the corresponding atom i β: 140,233,249 Here, α max is the highest atomic polarizability in the system and Δα is the difference between α max and the polarizability of the current atom α i β .…”

Overlooked electrolyte destabilization by manganese (II) in lithium-ion batteries

“…34,35 for Li + , PF 6 − , EC, DMC, and TFSI − , and Bradbrook et al 36 for Mn 2+ . The starting configurations were prepared utilizing fftool 37 and packmol 38 . The initial setup electrolytes were equilibrated at 393 K and 1 atm for 2 ns in NPT ensemble.…”

Effect of side chain modifications in imidazolium ionic liquids on the properties of the electrical double layer at a molybdenum disulfide electrode

“…Note that a trajectory of hundreds of nanoseconds is required to attain converged interfacial structures for these systems. The ILs are modeled with the recently developed Drude polarizable force field CL&Pol 46,50 with several refinements, whereas the electrode atoms are modeled as charge neutral particles with only LJ interactions. The details of force field and simulation conditions are given in section V. Near the electrodes, there are strong attractions between Drude particles and their images, which cause instability if a Drude particle comes too close to the electrode.…”