*3.4. Calculations*

Quantum-mechanical calculations using the B3LYP functional (DFT, the three-parameter exchange hybrid functional of Becke [63], and gradient-corrected correlation functional of Lee, Yang and Parr) [64] with the 6-311++G(d,p) basis set [65–67] were performed for full geometry optimizations with the *Gaussian* 16 Rev. C01 suite of program [68]. The structures were visualized by the MOLDEN program [69]. The potential energy distribution (PED) of the normal modes was calculated in terms of natural internal coordinates using the Gar2ped program [70]. Static periodic (solid-state) DFT calculations (DFPT *Density-Functional Perturbation Theory*) were performed in the CRYSTAL09 [71,72] software package using the B3LYP functional with the Grimme D2 dispersion correction [73] and 6-31G\*\* basis set [67]. The space groups and unit cell parameters of the considered crystals obtained in the experimental studies [27,28] were fixed and structural relaxations were limited to the position of atoms. Such approximation gives a reasonable description of the structure and spectroscopic features of intra- and intermolecular H-bonds of different types and strengths in molecular crystals [74–76]. The experimental atomic positions were used as the starting point in the periodic DFT computations. Periodic DFT computations were conducted for polymorphs α2 and β. The disordered α1-polymorph [27] is not suitable for periodic calculations. Since α1 and α2 polymorphs are quasi-isostructural, we assumed that their IR spectra are very close to each other. Therefore, only the IR spectrum of the second polymorph was considered in the present study. Periodic DFT computations of α2-polymorph led to the appearance of imaginary frequencies. This problem is usually solved by reducing the space symmetry of a crystal [77,78]. Reducing the space symmetry group to P1 allowed us to ge<sup>t</sup> rid of imaginary frequencies of α2-polymorph. An accurate interpretation of the experimental spectrum assumes knowledge of the relative stability of polymorphs α2 and β. This requires the calculation of the sublimation enthalpy, which is very cumbersome in the case of conformationally mobile molecules [79]. In addition, the error in the calculated values can reach 15 kJ/mol [80]. Such an accuracy of calculations is hardly suitable for describing "very close points in the crystal structure landscape" of crystals with conformationally mobile molecules, e.g., see [81].
