**4. Conclusions**

We investigate the effect of different electron-withdrawing and -donor groups on the energetics of the resonance assisted hydrogen bond in malondialdehyde. Our data indicate that classical contributions are far more important than exchange-correlation components as opposed to the notion that the stability of RAHBs occur mainly due to the delocalisation of *π* electrons. These groups exert a marked influence on the RAHB interaction energy, which in turn depends notably on the position of the EWG and EDG. Notably, both types of groups considerably strengthen the RAHB when they are bonded to the *β* carbon atom of malondialdehyde. We also addressed different methodologies to assess the interaction energy of RAHBs. In this regard, we showed how the examination of the energy density offers a good estimation of the IQA interaction energy and therefore of the RAHB energetics at a reduced computational cost.

**Supplementary Materials:** The following are available online, Table S1: IQA interaction energies for the investigated systems; Table S2: IQA interaction, formation, and HB interaction energies. Figures S1–S8: Structures of all compounds.

**Author Contributions:** All authors contributed equally to this work. All authors have read and agreed to the published version of the manuscript.

**Funding:** We gratefully acknowledge financial support from CONACyT/Mexico (grant 253776), PAPIIT/UNAM (project IN205118), and the Spanish MICINN (grant PGC2018-095953-B-I00). We are also grateful to DGTIC/UNAM (project LANCAD-UNAM-DGTIC-250) for computer time. Miguel Gallegos specifically acknowledges the Spanish MICIU for the predoctoral gran<sup>t</sup> FPU19/02903.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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