The chemical bond and solid-state physics

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Abstract
ADVANCES in quantum chemistry, as in any other field, need novel concepts to keep pace with the great increase in available experimental data and theoretical understanding. The concept of ionicity of the chemical bond is an example. For 35 years Linus Pauling's development of a definition of ionicity, based on heats of formation of binary crystals, was standard. Here I will describe a new definition of ionicity, derived from spectroscopically obtained transition energies between bonding and antibonding states of semiconductor crystals, and show how it compares with the older one. A new scale of ionicity, with which the ionic character of bonding in crystals can be predicted and measured, has led to improved understanding of such crystalline properties as lattice structure, heats of formation, elastic constants and nonlinear optical properties.