Trends in Two- and Three-Body Effects in Multiscale Clusters of Ionic Liquids

Applications of higher correlated levels of ab initio theory to condensed systems require a significant amount of computational resources. The recent development of the fragment molecular orbital (FMO) approach alleviates this issue by splitting the system into individual fragments and achieves the accuracy of the method by accounting for all possible two-body and three-body interactions. In this work a comprehensive application of the FMO approach in combination with a second order of Møller-Plesset perturbation theory method, MP2, is presented for multiscale clusters of ionic liquids such as [C₁mim]X, [C₁mpyr]X, [C₂py]X, and [NMe₄]X, where X = chloride and tetrafluoroborates, BF₄^-, with the clusters varying in size from 4, 8, 16, to 32 ion pairs. Reliable cutoff criteria for the inclusion of two-body and three-body interactions are identified for both HF energy and MP2 correlation energy to achieve the desired accuracy of 1 kJ mol^-1. The importance of two-body and three-body interactions in ionic liquids is also discussed.