Ion-pair binding energies of ionic liquids: can DFT compete with ab initio-based methods?

In this work, we examine the performance of a range of local and hybrid DFT functionals (including BLYP, PBE, PW91, B3P86, B3LYP, and TPSS), new generation DFT functionals (including KMLYP, BMK, M05, and M05-2X), and DFT functionals with the explicit empirical correction for dispersion interactions (including BLYP-D, PBE-D, and B3P86-D) in calculating ion-pair binding energies of pyrrolidinium-based ionic liquids, [C(n)mpyr][X] (n = Me, Et, n-Pr and n-Bu and X = Cl, BF(4), PF(6), CH(3)SO(3) (mesylate), CH(3)PhSO(3) (tosylate), N(CN)(2), and NTf(2)). Calculated IPBEs were compared to the results of the selected benchmark method, MP2/6-311+G(3df,2p). Modified MP2 methods such as SCS-MP2 and SOS-MP2 were also considered in the study. Errors of the DFT-based and ab initio-based methods in calculations of IPBEs of ionic liquids, trends in relative IPBEs, and basis-set superposition errors are discussed in depth. DFT functionals that can potentially be used to study binding energies of ionic liquids are identified.