QUANTUM-CHEMICAL AB-INITIO CALCULATIONS OF THE MAGNETIC INTERACTION IN ALKALITHIOFERRATES(III)

Quantum chemical ab initio calculations for two ternary thioferrate cr ystals, Na5FeS4 and Na3FeS3, have been performed. Using a cluster mode l approach with 250 optimized point charges for the single well-isolat ed complexes [FeS4](5-) and [Fe2S6](6-), the electronic and magnetic p roperties are investigated. In particular, the weak antiferromagnetic coupling of two half-filled Fe 3d shells in the dimeric complex is dis cussed within the active-electron approach. The complete active space self-consistent-field (CASSCF) wave functions with 10 active electrons in 10 active orbitals are analyzed by expectation values for the loca l spin and the local charge fluctuations at one Fe site, allowing for a quantitative discussion of interatomic and intraatomic nondynamical correlation. With CASSCF a clear deviation from the Heisenberg Hamilto nian is found with coupling constants ranging from J(4-->5)=-18 cm(-1) to J(0-->1)=-27 cm(-1), which is a factor 4 smaller than experiment. The need of going beyond the active-electron approach and correlating all ligand orbitals is demonstrated, and an approximative scheme based on increments derived from multireference configuration interaction a nd averaged coupled-pair functional calculations is proposed, which yi elds J(4-->5)=-65 cm(-1). (C) 1997 American Institute of Physics.