Title: Revisiting Imidazolium Based Ionic Liquids: Effect of the Conformation Bias of the [NTf$_{2}$] Anion Studied By Molecular Dynamics Simulations,
Abstract: We study ionic liquids composed 1-alkyl-3-methylimidazolium cations and
bis(trifluoromethyl-sulfonyl)imide anions ([C$_n$MIm][NTf$_2$]) with varying
chain-length $n\!=\!2, 4, 6, 8$ by using molecular dynamics simulations. We
show that a reparametrization of the dihedral potentials as well as charges of
the [NTf$_2$] anion leads to an improvment of the force field model introduced
by Köddermann {\em et al.} [ChemPhysChem, \textbf{8}, 2464 (2007)] (KPL-force
field). A crucial advantage of the new parameter set is that the minimum energy
conformations of the anion ({\em trans} and {\em gauche}), as deduced from {\em
ab initio} calculations and {\sc Raman} experiments, are now both well
represented by our model. In addition, the results for [C$_n$MIm][NTf$_2$] show
that this modification leads to an even better agreement between experiment and
molecular dynamics simulation as demonstrated for densities, diffusion
coefficients, vaporization enthalpies, reorientational correlation times, and
viscosities. Even though we focused on a better representation of the anion
conformation, also the alkyl chain-length dependence of the cation behaves
closer to the experiment. We strongly encourage to use the new NGKPL force
field for the [NTf$_2$] anion instead of the earlier KPL parameter set for
computer simulations aiming to describe the thermodynamics, dynamics and also
structure of imidazolium based ionic liquids.