Approximate Multiconfigurational Treatment of Spin-Coupled Metal Complexes.

The weak interaction between unpaired electrons in polynuclear transition-metal complexes is often described by exchange and spin polarization mechanisms. The resulting intrinsic multiconfigurational electronic structure for such complexes may be calculated with wave function-based methods (e.g., complete active space configuration interaction and complete active space self-consistent field), but computations become extremely demanding and even unfeasible for polynuclear complexes with a large number of open-shells. Here, several levels of selection of configurations and symmetry considerations that still capture the essential physics of exchange and spin polarization mechanisms are presented. The proposed approximations result in significantly smaller configuration interaction expansions and are equally valid for ab initio and semiempirical methods. Tests are performed in simple molecular systems and in small transition-metal complexes that cover a range of valence and charge states. In particular, superexchange contributions can be calculated to good accuracy using only single ionic excitations. Further reduction in the size of the configuration expansions is possible but restricts the description to low-lying spin ladders. The proposed configuration interaction schemes may be used to resolve space and spin symmetries in the calculation of electronic structures, exchange coupling constants, and other properties pertinent to polynuclear transition-metal complexes.