## 1 Introduction  
Terahertz (THz) spectroscopy stands as an established instrument for investigating molecular dynamics, with a special focus on biomolecules [1, 2, 3, 4, 5, 6]. The central part of the THz spectral region extends from 0.3 THz to 3 THz (wavenumbers 10-100 cm\({}^{-1}\)) and corresponds loosely to photon energies between 1 and 12 meV [1, 2], equal to the energy of large-scale molecular vibrations that are mediated by intermolecular bonds [4]. Covalent, ionic, or metallic bonds and, in some situations, even hydrogen bonds (H-bonds), take on the role of shaping the intramolecular arrangement of a molecule's constituent atoms. The intermolecular forces that hold biomolecules together in the crystal lattice are typically H-bonds or weaker forces such as van der Waals, dipole-dipole, or dispersion forces. The complicated interplay between intramolecular and intermolecular forces is responsible for low-frequency vibrations of molecular crystals and creates the shape of the THz absorption spectrum. At higher frequencies, the spectra are determined primarily by intramolecular forces [2; 7]. The THz vibrational modes are delocalized over a large number of atoms within the molecular crystal's unit cell and are impossible to simply assign to a particular stretch or bend mode, as is possible for infrared spectroscopy, in particular above 1500 cm\({}^{-1}\). This is not a disadvantage but represents one of the strengths of THz spectroscopy, which is able to provide information on the low-frequency collective motion of highly flexible organic molecules and distinguish, for example, among structural isomers [2]. In order to identify and analyze the low-frequency vibrational modes, ab initio simulations based on density functional theory (DFT) calculations are extremely useful. They were utilized to gain insights into the THz spectra of biological molecules. Moreover, these methods find frequent applications in the prediction of vibrational spectra for individual molecules, dimers, and clusters, and their usage is also on the rise for solid-state systems [4; 6; 7]. However, when dealing with solid-state systems, a significant challenge persists due to the intricate nature of performing periodic DFT modeling on extensive structures such as macromolecules and molecular crystals [8; 9]. In addition, it was shown that THz spectroscopy allows us to certify molecular structure and intermolecular H-bonds interactions within compounds, thus, offering a foundational basis for exploring emerging pharmaceutical cocrystal fields at the micro-molecular level [10]. The presence of H-bonds and weaker forces allows biomolecules to be stable and highly flexible in their natural environments [11]. This plays a significant role in biochemical reactions at the basis of life. Saccharides are carbohydrates or sugars that are major products of the photosynthesis of green plants. They are an essential structural component of living cells and function as the main source of energy that fuels various biochemical processes needed to maintain the life of animals and plants. Additionally, they play a relevant role in industrial applications, and they are used in the production of paper, food sectors, pharmaceuticals, and regenerative medicine [12]. Saccharides may be classified according to the number of monomeric units that comprise them. The compounds studied in this article are: (i) the monosaccharide glucose and galactose, both with C\({}_{6}\)H\({}_{12}\)O\({}_{6}\) molecular structure but with a difference in the direction of the hydroxyl group in the carbon number 4, C(4), of the carbon skeleton; (ii) the disaccharide lactose, made of galactose and glucose units, and cellobiose, made of two glucose units, both bound together by a glycosidic linkage and with the general formula C\({}_{12}\)H\({}_{22}\)O\({}_{11}\); (iii) the polysaccharide cellulose, an unbranched chain of D-glucose units linked also in this case by glycosidic linkages with (C\({}_{6}\)H\({}_{10}\)O\({}_{5}\))\({}_{n}\) formula (\(n\ \geq\ 100\)).