Abstract
Clusters of two ion pairs of imidazoliumbased ionic liquids were optimized with 43 different levels of theory, including DFT functionals and MP2based methods combined with varying Dunning's basis sets, and added dispersion corrections. Better preforming DFT functionals were then applied to clusters consisting of four ion pairs. Excellent performance of some DFT functionals for the two ion pair clusters did not always match that of the four ionpaired clusters despite interionic distances remaining constant between the optimized two and four ionpaired clusters of the same ionic liquid. Combinations of DFT functional and basis set such as ωB97XD/ccpVDZ, M062X/augccpVDZ, B3LYPD3/ccpVTZ, and TPSSD3/ccpVTZ gave excellent results for geometry optimization of two ionpaired clusters of imidazolium ionic liquids but gave larger deviations when applied to the four ionpaired clusters of varying ionic liquids. Empirical dispersion corrections were seen to be crucial in correctly capturing correlation effects in the studied ionic liquid clusters, becoming more important in larger clusters. Dunning's doubleζ basis set, ccpVDZ, is associated with the smallest root mean squared deviations for geometries; however, it also produces the largest deviations in total electronic energies. ωB97XD and M062X produced the best performance with the augmented version of this basis set. The tripleζ basis set, ccpVTZ, leads to the best performance of most of the DFT functionals (especially the dispersioncorrected ones) used, whereas its augmented version, augccpVTZ, was not seen to improve results. The combinations of functional and basis set that gave the best geometry and energetics in both two and four ionpaired clusters were PBED3/ccpVTZ, ωB97XD/augccpVDZ, and BLYPD3/ccpVTZ. All three combinations are recommended for geometry optimizations of larger clusters of ionic liquids. PBED3/ccpVTZ performed the best with an average deviation of 2.3 kJ mol1 and a standard deviation of 3.4 kJ mol1 for total electronic energy when applied to four ionpaired clusters. Geometries optimized with FMO2SRSMP2/ccpVTZ produced total energy within 2.0 kJ mol1 off the benchmark in two ionpaired clusters, with the ccpVDZ basis set performing unsurprisingly poorly with the same method. The error increased to 4.8 kJ mol1 on average in four ionpaired clusters, with the smallest RMSD deviations in geometries when compared to the benchmark ones. This study is the first report that investigated the performance of DFT functionals for two and four ionpaired clusters of a wide range of ionic liquids consisting of commonly used cations such as pyrrolidinium, imidazolium, pyridinium, and ammonium. It also identified the importance of assessing the performance of quantum chemical methods for ionic liquids on a variety of cationanion combinations.
Original language  English 

Pages (fromto)  67356753 
Number of pages  19 
Journal  Journal of Chemical Theory and Computation 
Volume  16 
Issue number  10 
DOIs  
Publication status  Published  13 Oct 2020 
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