Comparison of Force Fields on the Basis of Various Model Approaches-How To Design the Best Model for the [CnMIM][NTf2] Family of Ionic Liquids

• Published in: Chemphyschem 2013-10, Vol.14 (14), p.3368-3374
• Main Authors: Köddermann, Thorsten, Reith, Dirk, Ludwig, Ralf
• Format: Article
• Language: English
• Subjects: Cations; Chains; Chemistry; Exact sciences and technology; force fields; General and physical chemistry; GROW; Heat of vaporization; Ionic liquids; Ions; molecular dynamics; NMR; Nuclear magnetic resonance; Optimization; Parameterization; quantum mechanics; Real time; Simulation; Solutions; Solvation. Solvent properties; Thermodynamic properties; Water chemistry; Workflow
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201300486

In this contribution, we present two new united‐atom force fields (UA‐FFs) for 1‐alkyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide [CnMIM][NTf2] (n=1, 2, 4, 6, 8) ionic liquids (ILs). One is parametrized manually, and the other is developed with the gradient‐based optimization workflow (GROW). By doing so, we wanted to perform a hard test to determine how researchers could benefit from semiautomated optimization procedures. As with our already published all‐atom force field (AA‐FF) for [CnMIM][NTf2] (T. Köddermann, D. Paschek, R. Ludwig, ChemPhysChem­ 2007, 8, 2464), the new force fields were derived to fit experimental densities, self‐diffusion coefficients, and NMR rotational correlation times for the IL cation and for water molecules dissolved in [C2MIM][NTf2]. In the manual force field, the alkyl chains of the cation and the CF3 groups of the anion were treated as united atoms. In the GROW force field, only the alkyl chains of the cation were united. All other parts of the structures of the ions remained unchanged to prevent any loss of physical information. Structural, dynamic, and thermodynamic properties such as viscosity, cation rotational correlation times, and heats of vaporization calculated with the new force fields were compared with values simulated with the previous AA‐FF and the experimental data. All simulated properties were in excellent agreement with the experimental values. Altogether, the UA‐FFs are slightly superior for speed‐up reasons. The UA‐FF speeds up the simulation by about 100 % and reduces the demanded disk space by about 78 %. More importantly, real time and efforts to generate force fields could be significantly reduced by utilizing GROW. The real time for the GROW parametrization in this work was 2 months. Manual parametrization, in contrast, may take up to 12 months, and this is, therefore, a significant increase in speed, though it is difficult to estimate the duration of manual parametrization. Lower the force field! A test to determine how researchers can benefit from semiautomated optimization procedures if using parametrized classical force fields is performed. Two united‐atom force fields for 1‐alkyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide [CnMIM][NTf2] (n=1, 2, 4, 6, 8) ionic liquids are developed that save simulation time and disk space. All simulated properties are in excellent agreement with the experimental values.