THE NATURE OF MULTIPLE BONDS .1. SIGMA,PI-BONDS VS BENT BONDS, A COMPUTATIONAL SURVEY

We examine the nature of multiple bonds within an independent-particle picture, climbing a hierarchy of approximations to the full generaliz ed valence bond (GVB) description, the most general wave functions for which an independent particle interpretation of the bonding is possib le. We begin with a systematic computational survey of molecules with multiple bonds, comparing sigma, pi and bent bond descriptions, using the strongly orthogonal perfect-pairing (SOPP) approximation to the GV B method. Although in several cases the bent-bond model is energetical ly favored using SOPP-GVB wave functions, in a number of other cases t he usually accepted sigma, pi bond description is energetically favore d. In order to obtain a definitive answer to the long-standing questio n of the better orbital description of the bonding, we conclude that t he full GVB theory must be employed without restrictions. We outline a procedure by which generalized valence bond wave functions can be rig orously obtained without spin coupling or orbital orthogonality constr aints and apply it to a sequence of molecules directed toward finding the best qualitative description of the bonding in multiple bonds: sig ma, pi bonds or bent bonds? Our calculations lend support to Slater's and Pauling's sixty-year-old conjectures, finding the bent bond descri ption energetically superior to the symmetry-restricted sigma, pi bond representation in the theoretical limit of wave functions able to mea ningfully address this question.