DFT calculations have been carried out with the Gaussian 09 suite of programs . Using the density functional theory with the hybrid B3LYP functional and with Grimme's B97D functional including dispersion . All monomeric ion calculations at the B3LYP and ω97X-D levels have been carried out with the 6-311++G(d,p), 6-311++G(3df,2dp), and DGDZVD basis sets. All structures have been fully optimized both in the gas phase and using the polarizable continuum model (PCM) implemented in the Gaussian program. Electrostatic interactions of the ion pair with surrounding ions were effectively accounted for by introducing apparent dielectric constant ε for the RTIL environment. Previous reports show that this approach, originally introduced for quadrupolar solvents, such as benzene and supercritical carbon dioxide, was able to describe solvation effects in nondipolar solvents . In the present study, we employed acetonitrile (ε = 36) to model [TFSI] in the PCM framework. The use of polarizable continuum model (PCM) is based on MD simulation results that imidazolium-based ILs show somewhat larger solvatochromic shifts. Each structure has been individually optimized for each method. Natural bond orbital analysis (NBO) analyses for all the ILs was made at ω97X-D levelswith the DGDZVD basis set using the polarizable continuum model (PCM). This analysis helps us to identify specific orbital interactions, NBO charges and also to obtain the descriptions of orbital hybridization. Vibrational frequencies of all the optimized structures have also been calculated to ensure that the optimized structure represents the true minimum. The absence of imaginary vibrational frequency confirms that the optimized geometry represents the stable structure. Basis set superposition error (BSSE) correction was not employed as its contribution to the interaction energy for an ionic salt are already reported to be insignificant .