Understanding energetics of bond formation and bond rotation with density functional theory and valence bond theory

Covalent bonding and noncovalent interactions are fundamental concepts in chemistry, biology, and related fields, yet the energetic factors driving bond formation and bond rotation remain a subject of ongoing debate in the literature because different theoretical frameworks might provide different insights. In this study, we examine the energetics of bond formation and bond rotation with both density functional theory and valence bond theory. Our analysis spans a wide range of systems, including 40 diatomic molecules; six energy profiles for Ar2, H2, F2, NaF, (H2O)2, and Na2 molecules; and six rotational barriers for CH3CH3, CH3NH2, CH3OH, H2O2, NH2NH2, and NH2OH. We find that (i) electrostatic energy is the dominant contributor in most cases; (ii) within the electrostatic terms, nuclear-electron attraction is often, but not always, the leading contributor; (iii) during bond formation, different interactions dominate at different stages; and (iv) in multi-electron systems, steric effects consistently contribute positively to the total energy decrease due to the spatial constraints imposed by the Pauli exclusion principle. These findings are consistently supported by both theories. Overall, this work should help bridge a critical knowledge gap by providing a unified perspective on the energetics of fundamental bonding processes.