The quantum mechanical description of molecules has shown that atoms in molecules differ from free atoms, viz., with regard to their electronic structures. Quantum mechanics has also suggested however that atomic subunits, quasi-atoms, are preserved in molecular electronic wave functions. Since the energy of a quasi-atom is higher than that of the free atom, binding energies embody energy increases due to the formation of quasi-atoms and larger energy decreases due to the interactions between the quasi-atoms. The ab initio quantification of these qualitative inferences requires the rigorous extraction of quasi-atoms from molecular electronic wave functions. Drawing a quantitative formulation of the quasi-atom concept out of ab initio wave functions has presented a major challenge for a deductive ab initio energy resolution. In all past ‘energy decomposition analyses’, intuitively formulated quasi-atoms were postulated as intermediate states between the nonbonded constituent atoms and the bonded system. No intermediate wave functions are introduced in the present analysis. Rather, the foundation is the rigorous demonstration that atomic subunits and interactions between them are embedded in the structure of electronic wave functions. The intrinsic quasi-atomic organization of the wave function is exhibited by appropriate transformations of the molecular orbital basis and the configurational representation, using basis set independent criteria. The molecular wave function is expressed in terms of quasi-atomic molecular orbitals that maximize the overlap between molecular orbital subspaces and free-atom orbital spaces. Thereby, the molecular wave function becomes the superposition of a wave function representing non-bonded juxtaposed quasi-atoms and a wave function describing interatomic electron migrations that create bonds through electron sharing. The binding energy is a sum of contributions due to quasi-atom formation, electrostatic interactions and interatomic interferences caused by electron sharing. The contributions are further resolved according to orbital interactions. Covalent bonding results from the kinetic energy lowering due to interferences between quasi-atomic orbitals. Reference: A. C. West, M. W. Schmidt, M. S. Gordon, K. Ruedenberg J. Phys. Chem, A, (2017) 121, 1086−1105. DOI: 10.1021/acs.jpca.6b10911 Work supported by the National Science Foundation and the U.S. Department of Energy

Intrinsic resolution of molecular electronic wave functions and energies in terms of quasi-atoms and their interactions

254th National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.