The quantum chemical concepts developed by the pioneers of the 1930’s identified fundamental physical synergisms involved in chemical bonding. Basic qualitative insights were that atoms in molecules differ from free atoms and that the interatomic interactions are of two kinds: ‘classical’ coulombic and ‘nonclassical’ resulting from the sharing of electrons between atoms. Understanding bonding requires therefore understanding the energy increases due to the formation of ‘quasi-atoms’ as well as the larger energy decreases due to the interactions between the quasi-atoms. Since ab initio electronic wave functions and energies are complex, theoretically sound interpretations require ‘energy decomposition analyses’ of commensurate complexity in order to untangle the physical synergisms that are embedded in rigorous wave functions. A pivotal part is the formulation of the definitions and constructs that generate the “atoms in the molecule” because it determines the quantification of the competition between the intra-atomic energy increases and the interatomic bond forming energy decreases. Traditionally, this issue has been sidestepped by postulating assumed atomic valence states in molecules. However, if atoms are indeed embedded in the quantum mechanical treatment, then the actual electronic molecular wave function must contain a definite intrinsic quasi-atomic organization. The distinctive foundation of the present work is the rigorous demonstration that, by suitable transformations of the molecular electronic wave function, the intrinsic quasi-atoms and the interactions between them are brought to light. The molecular wave function is exactly expressed in terms of quasi-atomic orbitals, which are somewhat deformed free atom orbitals. Through this expression, the energy is additively resolved into contributions from the quasi-atoms, from coulombic interactions, from interactions due to electron sharing, and from interactions due to interatomic entanglements. All steps of this intrinsic energy decomposition are based on criteria that are basis set independent and free of biasing input information. The intrinsic energy decomposition analysis has been applied to a variety of molecules. J. Chem. Phys. 139, 234107, 1-19 (2013), J. Phys. Chem. A 119, 10360−10367 (2015) J. Phys. Chem, A 121, 1086−1105 (2017) Supported by U.S. National Science Foundation and U.S. Department of Energy

Atoms and bonds in molecular electronic wave functions: An intrinsic analysis

255th National Meeting (2018)

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