Bent-bond versus separated-bond models: A spin-coupled survey for a few organic and inorganic systems

Fully-variational spin-coupled calculations on various systems (ScCH2+, TiC H2+, C2H2, N-2, and C2H4) are used to illustrate factors which favor either the bent bond model or the symmetry separated (sigma + pi) model of multip le bonding. For systems that are not well described by a single mode of spi n coupling, the degree of flexibility in the total spin function can be mor e important than the number of degrees of freedom in the orbitals. Furtherm ore, restricting the number of correlated electrons has a more deleterious effect on the spin flexibility of the sigma + pi model than on its bent bon d alternative. (C) 1999 John Wiley & Sons, Inc.