Correcting Symmetry Problems in Electronegativity Equalization and Errors in Conceptual DFT

What is it about?

Electronegativity is the foremost ordering concept in the qualitative description of chemical phenomena. In addition to its qualitative aspects, the electronegativity concept serves to map different bond types (e. g., covalent, ionic, polar) on a single quantitative scale. It is particularly important for the fast assignment of bond polarities by electronegativity equalization. An order of magnitude improvement of the accuracy of electronegativity equalization is achieved by replacing the ground-state electronegativity by a new charge dependent pair-sharing potential, the valence-pair-affinity, which is equilibrated in bonds. The valence-pair-affinity is a measure for the potential of an atom-in-the-molecule (or functional group) to attract an electron pair in a sharing competition with another atom (or group) in the molecule. The failures of ground-state electronegativity equalization and operational chemical potential equalization (CPE) are understood and eliminated. The performances of the valence-pair equilibration (VPEq) and the operational chemical potential equalization are compared for 89 molecules with very diverse bond characters, including the "exotic" dimers Be_2, Mg_2, B_2, C_2 and Mn_2. The accuracy of VPEq is about 9 times better than that of operational CPE. Without requiring ad hoc calibrations, the VPEq bond polarities agree very well with results of state-of-the-art population analyses, and charges derived from vibrational spectra. A paradigm shift emphasizing valence states seems in order for conceptual density functional theory. Electronegativity and the chemical potential should be regarded as separate properties.

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Why is it important?

Guided by the Wigner-Witmer symmetry rules, the valence-pair-affinity correctly extends Mulliken´s atomic definition of electronegativity to bonds in molecules, where it is accurately equilibrated. The present work challenges conceptual Density Functional Theory, which has to remain under debate. Electronegativity and the electronic chemical potential should be regarded as two separate and distinct properties. To resolve the shortcomings of the operational CPE, the operational definition of chemical potential has to be significantly modified by adapting it to the Wigner-Witmer symmetry constraints, which are incompatible with the current ground-state paradigm of conceptual Density Functional Theory.