Study of the Electronic Structure of Molecules. I. Molecular Wavefunctions and Their Analysis

In this paper we have summarized the complete multiconfiguration self-consistent-field theory (CMC SCF LCAO MO or CMC for short) as given from the analysis of Veillard and the author (published elsewhere). In the analysis of the CMC theory it is shown that the CMC technique allows determination of the pair and pair—pair correlation energy and retains the physical simplicity of the Hartree—Fock functions. The CMC theory differs from usual many-body techniques in so far as it does not take the Hartree—Fock energy as zero-order energy. It has been shown that the CMC wavefunction can be cast in the form of a new type of ``single determinant'' (the ``CMC determinant'') with elements which are different from the standard elements of the Slater determinant. The existence of the CMC determinant provides the formal justification for the use of a single determinant in semiempirical work aiming at exact prediction. The analysis of the CMC wavefunctions is then continued in terms of a population analysis which to a large extent follows Mulliken's work. Then the energy expression of the SCF MO and CMC techniques are analyzed in terms of bond-energy diagrams. These are special partitions of the total energy with immediate reference to structural chemistry. Finally, the bond-energy diagrams are briefly discussed in their relation to bond-energy transferability, vibrational analysis, and other chemical phenomena. This paper introduces both the notation and the terminology we shall use in describing all-electron computations for the NH3, HCl reaction surfaces, and for pyrrole, pyridine, pyrazine, fluorobenzene, benzene, and a few other molecules presented in the following papers of this series.