Atoms and Bonds as Synergisms of Interacting Electrons and Nuclei. The Origin of Chemical Bonds in Polyatomic Molecules

In quantum mechanical elucidations, the molecular energy of formation from the free atoms is an aggregate of cooperativities (synergisms) of orbital interactions. Frequently, such synergisms are deduced from model wave functions, which are assumed in addition to the actual molecular wave function. By contrast, the present analysis determines synergisms that are embedded in the actual molecular wave function, without arbitrary assumptions. These synergisms are exposed by a resolution of the molecular energy based on intrinsic transformations of the actual molecular wave function. The resolution identifies the modified atoms in the molecule as those quasi-atomic substructures of the wave function that maximally overlap with the free-atom wave function. Bonding results from electron sharing between these quasi-atoms. The antibonding energy increase from the free atoms to the quasi-atoms and the bonding energy lowering through electron-sharing between the quasi-atoms are resolved in terms of synergisms involving quasi-atomic orbitals. The present formulations are based on our previous analyses. The resulting "intrinsic energy decomposition analysis" (IEDA) is quantitatively exemplified by application to the ethane molecule. The IEDA shows that the total theoretical molecular energy of formation is essentially caused by electron sharing in the C-C bond and the six C-H bonds. The bonding energy lowering in each of these bonds is almost entirely due to the lowering of the kinetic energy as a result of the interference of the bonded quasi-atomic orbitals. The results of the analysis suggest that arbitrarily assumed model wave functions do not reliably recover the intrinsic physics of molecule formation.