AB-INITIO STUDY OF THE MAGNETIC COUPLING IN NA6FE2S6

For the ternary thioferrate crystal Na6Fe2S6, ab initio quantum chemic al calculations using a cluster model ansatz have been performed to ex amine the magnetic coupling of the two half-filled Fe 3d shells in the isolated dimeric [Fe2S6](6-) complexes having the structure of edge-l inked double tetrahedra. The active-electron approach using complete a ctive space configuration interaction (CASCI) with 10 electrons in 10 orbitals yields the multiplet splitting of a two-center Heisenberg Ham iltonian with an antiferromagnetic coupling constant J = -19 cm(-1), w hich is by a factor of 5 smaller than the experimental value. Correlat ion effects are essential for the magnetic coupling, as the applicatio n of multireference second-order Moller-Plesset perturbation theory ba sed on the CIPSI algorithm (CAS-2nd) and the recently proposed differe nce-dedicated CI method lead to values J(4-->5) = -158 and -66 cm(-1), respectively, which clearly agree better with experiment. The differe nt electronic contributions to the chemical bonding in the binuclear t ransition metal complex have been investigated using the constrained s pace orbital variation method.