The origins of stabilization in the short strong hydrogen bonds commonly referred to as “resonance-assisted” (RAHB) have been revisited using the modern valence-bond theory, the hybrid variational–perturbational interaction energy decomposition scheme and atoms in molecules (AIM) analysis. Dimers of carboxylic acids and amides have been chosen as the model structures for intermolecular RAHBs, while for the intramolecular case malondialdehyde and its substituted derivatives have been selected. The estimated (negligible) resonance stabilization energies and relative magnitudes of interaction energy components indicate that the origin of stabilization in the studied complexes is charge-delocalization. Although in the case of intramolecular RAHBs the resonance effects are much more pronounced, still they are a relatively minor contribution to the total stabilization energy. In fact, the estimated resonance stabilization energies diminish with an increasing strength of the hydrogen bond (as indicated by AIM and structural descriptors).

The nature of interactions in the molecular crystal of urea is analyzed in terms of the interaction energy decomposition. The influence of the electron correlation effects was estimated on the basis of the calculated second order Møller–Plesset corrections and their analysis. In the crystal, the urea molecules form infinite ribbons which reveal strong cooperative effects. The hydrogen-bonded interactions of the orthogonal ribbons do stabilize the crystal, whereas interactions between parallel tapes are repulsive. The stability of the crystal structure is determined by a subtle balance between these two types of interactions. Although, the electron correlation effects are stabilizing, their contribution is rather small in comparison with the total interaction energy.