In order to give a visual understanding of the cation-anion interaction before designing initial geometries for the ion pairs (dimers), the most stable geometries of the isolated [EMI] + cation, As can be seen from Tables S1-S4, bond lengths, angles and dihedrals of the conformers calculated using the ω97X-D level of theory are in good agreement among the various basis sets and the ω97X-D/6-311++(3df,2dp) level of theory and basis set gives calculated results that best agree with those in crystals . On the other hand, bond lengths calculated using the B3LYP level of theory show some variations larger than those in the crystals as well as than that of ω97X-D results. These imply that the wB97X-D functional with a larger basis set with polarization functions show good performance for such molecules involving fluorine or sulfur atoms . The energy of the cis-[TFSI] -conformer relative to that of the trans-[TFSI] - conformer calculated using ω97X-D/6-311++(3df,2dp) levels of theory and the basis set and in the presence of dielectric continuum medium employed here is 3.025 kJ mol -1 . The small value, relative to the RT value at 298 K implies that the conformers are present in equilibrium. The dipole moment of the conformers is also shown. According to the ω97X-D/6-311++(3df,2dp) levels of theory and basis set and in the presence of dielectric continuum medium, the dipole moment of the cis-[TFSI] -(µ = 6.0640) conformer is significantly larger than that of the trans-