Efficient Parametrization of Force Field for the Quantitative Prediction of the Physical Properties of Ionic Liquid Electrolytes

The
prediction of transport properties of room-temperature ionic
liquids from nonpolarizable force field-based simulations has long
been a challenge. The uniform charge scaling method has been widely
used to improve the agreement with the experiment by incorporating
the polarizability and charge transfer effects in an effective manner.
While this method improves the performance of the force fields, this
prescription is ad hoc in character; further, a quantitative prediction
is still not guaranteed. In such cases, the nonbonded interaction
parameters too need to be refined, which requires significant effort.
In this work, we propose a three-step semiautomated refinement procedure
based on (1) atomic site charges obtained from quantum calculations
of the bulk condensed phase; (2) quenched Monte Carlo optimizer to
shortlist suitable force field candidates, which are then tested using
pilot simulations; and (3) manual refinement to further improve the
accuracy of the force field. The strategy is designed in a sequential
manner with each step improving the accuracy over the previous step,
allowing the users to invest the effort commensurate with the desired
accuracy of the refined force field. The refinement procedure is applied
on N,N-diethyl-N-methyl-N-(2-methoxyethyl)­ammonium bis­(trifluoromethanesulfonyl)­imide
(DEME-TFSI), a front-runner as an electrolyte for electric double-layer
capacitors and single-molecule-based devices. The transferability
of the refined force field is tested on N,N-dimethyl-N-ethyl-N-methoxyethoxyethylammonium
bis­(trifluoromethanesulfonyl)­imide (N112,2O2O1-TFSI). The refined force field is found to be better
at predicting both structural and transport properties compared to
the uniform charge scaling procedure, which showed a discrepancy in
the X-ray structure factor. The refined force field showed quantitative
agreement with structural (density and X-ray structure factor) and
transport propertiesdiffusion coefficients, ionic conductivity,
and shear viscosity over a wide temperature range, building a case
for the wide adoption of the procedure.