Many-Electron Theory of Atoms and Molecules. I. Shells, Electron Pairs vs Many-Electron Correlations

TLDR

Configuration‑interaction theory shows that multiple excitations arise from simultaneous binary electron collisions rather than higher‑order multi‑electron collisions. The authors develop a theory to identify key correlation features, construct a quantitative N‑electron scheme for systems like He and H₂, and assess its impact on chemical concepts, semi‑empirical models, and shell structure. The non‑perturbative theory employs the r₁₂‑coordinate method, enabling treatment of N‑electron systems without series expansions. The study demonstrates that pair correlations dominate in atoms and molecules, and validates the theory on Be, LiH, and boron.

Abstract

A theory is developed (a) to see what the physically important features of correlation in atoms and molecules are; (b) based on this to obtain a quantitative scheme for N-electron systems as in He and H2; (c) to see what happens to the ``chemical'' picture, to semiempirical theories, and to shell structure, when correlation is brought in. It is shown why, unlike in an electron gas, in many atoms and in molecules mainly pair correlations are significant. In configuration-interaction, multiple excitations arise not as three or more electron ``collisions,'' but as several binary ``collisions'' taking place separately but at the same time. The validity of theory is shown on Be, LiH, and boron. The theory does not depend on any perturbation or series expansion and the r12-coordinate method can now be used for an N-electron system as in He and H2.