Molecular interactions in 1,3-dimethylimidazolium-bis(trifluromethanesulfonyl)imide ionic liquids are studied by employing the ab initio Hartree-Fock and density functional methods. Different conformers are simulated with respect to molecular interactions between cation and anion. Manifestation of these molecular interactions in the vibrational spectra is discussed. Direction of frequency shifts of the normal vibrations relative to those in free anion and cations are explained by calculating difference electron density coupled with molecular electron density topography.