Many-Electron Theory of Nonclosed-Shell Atoms and Molecules. II. Variational Theory

The theory developed in I is generalized into a variational many-electron theory by making the first-order correlation processes in the wavefunction ``exact to all orders'' and by including the effects of unlinked clusters. Equations for obtaining the significant correlation effects are derived by the variation—perturbation approach. Symmetry properties of the correlation functions and a semiempirical theory based on the form of the variational energy are also discussed. The examples of lithium, 1S-carbon, and C2H4+ illustrate some aspects of the theory.