**4. Conclusions**

The nature of intermolecular hydrogen bonds in exemplary compounds derived from benzamide, quinoline and benzoic acid groups was investigated. Quantum-chemical simulations were performed in vacuo and in the crystalline phase allowing a more indepth analysis of the non-covalent interactions present in the chosen compounds for the current study: 2,6-difluorobenzamide, 5-hydroxyquinoline and 4-hydroxybenzoic acid. The computed power spectra of the atomic velocity reproduced the spectroscopic features of the investigated compounds indicating regions with O-H, N-H and C-H stretching. The comparison was made between the gas and crystalline phases, indicating the formation of the intermolecular hydrogen bonds. The metric parameter analysis based on the Car– Parrinello molecular dynamics results showed that the proton transfer phenomenon occurs for 4-hydroxybenzoic acid in the crystalline phase. Based on the results of the PIMD method, it was noted that the inclusion of quantum effects in the description of hydrogen bonds is important for strong interactions. Furthermore, theoretical investigations on the basis of static approaches (QTAIM and SAPT methods) revealed the strength of the noncovalent interactions as well as energy components. Hydrogen bonding energies estimated according to Espinosa's formula indicate that, in the studied dimers, the interaction is stronger in 4-hydroxybenzoic acid. Finally, the SAPT results provided a detailed look inside the energy components of the intermolecular interactions. It was shown that the dispersion and the induction contributions to the interaction energy are decisive factors in the intermolecular hydrogen bonds studied herein. The HBs strength is strongly correlated with their covalency.

**Author Contributions:** Conceptualization, K.W. and A.J.; methodology, K.W. and A.J.; validation, K.W.; formal analysis, K.W.; investigation, K.W. and A.J.; writing—original draft preparation, K.W.; writing review and editing, K.W. and A.J.; visualization, K.W.; supervision, A.J.; project administration, K.W. and A.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors thank Jarosław J. Panek for the scientific discussions. The authors gratefully acknowledge the Academic Computing Centre Cyfronet-Kraków (Prometheus supercomputer, part of the PL-Grid infrastructure) as well as the Pozna ´n Supercomputing and Networking Center (PSNC) for generous grants of computer time and facilities. In addition, the ARCHER2 UK National Supercomputing Service (https://www.archer2.ac.uk) is acknowledged for generous CPU time and facilities in the framework of the DECI–17 access program.

**Conflicts of Interest:** The authors declare no conflict of interest.
