  We present an ab initio study of electronic correlation effects in a
molecular cluster derived from the hexanuclear ferric wheel
[LiFe6(OCH3)12-(dbm)6]PF6. The electronic and magnetic properties of this
cluster have been studied with all-electron Hartree-Fock, full-potential
density functional calculations and multi-reference second-order perturbation
theory. For different levels of correlation, a detailed study of the impact of
the electronic correlation on the exchange parameter was feasible. As the main
result, we found that the influence of the bridge oxygen atoms on the exchange
parameter is less intense than the influence of the apical ligand groups, which
is due to the geometry of the cluster. With respect to the cluster model
approach, the experimental value of the exchange parameter was affirmed.
