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

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.