BENT VERSUS SIGMA-PI-BONDS IN ETHENE AND ETHYNE - THE SPIN-COUPLED POINT-OF-VIEW

The bent-bond (equivalent) and sigma-pi orbital models for the carbon-carbon double and triple bonds in ethene and ethyne are compared for a whole hierarchy of one-configuration wave functions, starting from the Hartree-Fock wave function, passing through the generalized valence-bond wave function with perfect-pairing and strong-orthogonality constraints, and finishing with the most general wave function based on a single orbital product: the spin-coupled (SC) wave function. The results are also compared with those from complete-active-space self-consistent field (CAS SCF) calculations performed using equivalent divisions of the electrons into core and valence subsets. Equivalent orbitals have been found to yield lower-energy discriptions of the carbon-carbon multiple bonds when applied in the SC framework, where the full spin space is utilized for the valence electrons and no orthognality constraints are imposed. However, the energy differences between this model and that using sigma and pi orbitals are found to be much smaller than the differences between the energies of the corresponding SC and CAS SCF wave functions. Thus, from an energetical point of view, both constructions provide an equally good starting point for the treatment of correlation effects beyond the one-configuration approximation.