Spin correlation in π-electron systems from spin-coupled wavefunctions. I. Theory and first applications

Abstract

We present a new method for the analysis of spin correlation in molecular systems. The method is based on the spin-coupled theory of molecular electronic structure, the modern single-configuration generalization of classical valence bond theory. The analysis consists in performing a decoupling of the 〈Š2〉 expectation value into local contributions arising from electron spins in different subgroups of spin-coupled orbitals. The method is then illustrated by the application to the lowest singlet and triplet states of two simple π-electron systems - octatetraene and benzene. A spin polarization mechanism is highlighted, which together with the classical valence bond concept of resonance provides the key to the description of spin correlation in these and related systems.