## 1 Introduction  
Ionic liquids (ILs) have attracted huge attention in the last years for their potential applications in a large variety of fields: as solvents [1], sorbents of dangerous or hazardous gases [2], components of microemulsions [3] or extraction [4] or microextraction systems [5], vehicles for drug delivery [6], thermally responsive materials [7], activation agents for biocatalysis [8] or components of electrolytes for electrochemical devices [9, 10]. At the basis of the versatility of ILs, there is the possibility to combine a large number of anions and cations to optimize an IL to the desired property. Usually, the cations are organic ions, such as imidazolium, pyrrolinium, alkyl ammonium or phosphonium, while the anions can be inorganic or organic ions, such as tetrafluoroborate (BF\({}_{4}\)), hexafluorophosphate (PF\({}_{6}\)), dicyanamide (DCA), tetracyanathematanide (B(CN)\({}_{4}\)), trifluorometanesulfonate (TfO, also known as triflate), bis(trifluorometanesulfonyl)imide (TFSI) or bis(fluorosulfonyl)imide (FSI). In addition, it is possible to combine two or more ILs together [11] or with solvents, so that an incredible number of combinations are envisaged. Vibrational spectroscopies, such as infrared or Raman, are powerful investigation techniques of the internal structure of ILs and of the possible phase transitions that ILs can undergo as a function of temperature or pressure. Many experimental and computational studies in this field have been summarized in a recent review [12]. The exploitation of temperature and pressure dependence of physical properties is not equivalent from the thermodynamic point of view because pressure mainly induces a variation of density, whereas temperature varies kinetic energy and density simultaneously [13; 14]. Indeed, many ILs display different behaviors as a function of temperature or pressure [15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26]; for example, some ILs that crystallize upon cooling transform, instead, into a glass state at high pressure. It must be noted that when pressure is applied, a compression is induced, the intermolecular distances decrease and the material can experience the repulsive part of the intermolecular potential, which is not accessible by a change of temperature [27]. Moreover, useful information is obtained by the comparison of experimental data with computational results, which are easily obtained in the case of ionic liquids. A high valuable consequence of this approach is that calculations can indicate strategies for the synthesis of ILs with specific properties, avoiding a simple \"trial and error\" process. Several reviews [28; 29] reported on the progress of multiscale calculations starting from the knowledge of the single ions composing an IL to calculate macroscopic physical quantities, such as viscosity, decomposition temperatures and ionic conductivity, or to predict how to design a reactor or a whole process based on ILs. Density Functional Theory (DFT) calculations are commonly used to investigate the molecular structure of compounds or ions and have been largely used for the study of ILs [12]. One of the results of these kinds of calculations are the vibrational frequencies and the infrared and Raman intensities, which can be easily compared to experiments. For many years, DFT studies were devoted to the investigations of single cations or anions and to the occurrence of their conformers [30; 31; 32; 33; 34]. More recently, calculations performed on ionic couples or on a limited number of ionic couples have become more easily accessible from a computational point of view, providing important information about the interactions between anion and cation and among ionic couples [35; 36]. In particular, concerning intermolecular interactions, interesting information has been obtained about the occurrence of hydrogen bonding [37; 38; 39; 40; 41]. Subtle changes in the vibrational spectra can be associated to specific interactions.