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 th e Hartree-Fock wave function, passing through the generalized valence- bond wave function with perfect-pairing and strong-orthogonality const raints, and finishing with the most general wave function based on a s ingle orbital product: the spin-coupled (SC) wave function. The result s are also compared with those from complete-active-space self-consist ent field (CAS SCF) calculations performed using equivalent divisions of the electrons into core and valence subsets. Equivalent orbitals ha ve been found to yield lower-energy discriptions of the carbon-carbon multiple bonds when applied in the SC framework, where the full spin s pace is utilized for the valence electrons and no orthognality constra ints are imposed. However, the energy differences between this model a nd that using sigma and pi orbitals are found to be much smaller than the differences between the energies of the corresponding SC and CAS S CF wave functions. Thus, from an energetical point of view, both const ructions provide an equally good starting point for the treatment of c orrelation effects beyond the one-configuration approximation.