Spin-Coupled Generalized Valence Bond Theory: An Appealing Orbital Theory of the Electronic Structure of Atoms and Molecules

This chapter outlines the basic elements of Spin-Coupled Generalized Valence Bond (SCGVB) theory, including the structure of the SCGVB wavefunction as well as its optimization and analysis. The SCGVB wavefunctions accounts for non-dynamical electron correlation yet still provides a compelling orbital description of the electronic structures of the ground and excited states of molecules and of the electronic mechanisms of chemical reactions. The insights provided by SCGVB theory are illustrated using a representative set of examples: the “traditional” HCN molecule, and its fragmentation into CH and N; the formation of “hypervalent” SF4 and SF6 from SF(n–1) + F; resonance/antiresonance in benzene and tropylium cation (C7H7+ ); the excited states of benzene, CH and O3/SO2 ; and the reaction of ground state methylene with H2 . The effects of dynamical electron correlation, which must be included to make quantitative predictions of molecular properties, can be taken into account by using the SCGVB wavefunction expressed in natural orbitals as the reference wavefunction in traditional multireference configuration interaction calculations.