DENSITY-FUNCTIONAL THEORY OF SPIN POLARIZATION AND SPIN COUPLING IN IRON-SULFUR CLUSTERS

This chapter discusses recent progress toward the development of a unified picture of the electronic structures and spin interactions of iron–sulfur and related systems; these concepts provide a close connection between a spin Hamiltonian description and a more detailed orbital picture of the electron distribution. The iron–sulfur proteins and synthetic analogs are challenging systems for quantum mechanical methods because they contain a large number of electrons and because spin polarization and spin coupling are essential features of the complexes. Standard approaches of ab initio quantum chemistry start from a spin-restricted picture, which is poorly adapted to problems involving high-spin transition metal centers. For this reason, a combination has been developed of broken symmetry and spin-unrestricted methods that is particularly well adapted to study spin-polarized and spin-coupled systems. These ideas are well adapted for use with density-functional methods. In the chapter, the basic ideas are developed of this approach, using a perturbation theory formalism to rationalize the spin Hamiltonian and energy-splitting formulas that should be appropriate for spin-coupled transition metal clusters. © 1992, ACADEMIC PRESS, INC.