Orbital correlation diagrams based on multiconfigurational variation of moments I. Theory

A multiconfigurational variation of moments ( MCM ) is presented to determine correlated orbital energies. Appropriate multiconfigurational one‐particle operators are introduced starting for closed‐shell systems from the restricted Hartree–Fock ( RHF ) scheme and for open‐shell system from the unrestricted Hartree–Fock ( UHF ) scheme. The advantage of these operators is the linear form of the correlation energy in the configuration interaction ( CI ) coefficients. They obey HF analogous pseudoeigenvalue equations and are therefore a generalization of the HF operator. In principle, the scheme can be extended to a multiconfiguration self‐consistent field ( MCSCF ) procedure for moments. In contrast to the SCF formalism they allow an orbital description of degeneracy and quasidegeneracy of the highest occupied ( HOMO ) and lowest unoccupied ( LUMO ) molecular orbital. With these operators, funnels of thermally forbidden Woodward–Hoffman reactions, as well as dissociations, can be described in a physically meaningful fashion by orbitalcorrelation diagrams.