Quantum chemical ab initio calculations for two ternary thioferrate crystals, Na5FeS4 and Na3FeS3, have been performed. Using a cluster model approach with 250 optimized point charges for the single well-isolated complexes [FeS4](5-) and [Fe2S6](6-), the electronic and magnetic properties are investigated. In particular, the weak antiferromagnetic coupling of two half-filled Fe 3d shells in the dimeric complex is discussed 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 local 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 Hamiltonian 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 and averaged coupled-pair functional calculations is proposed, which yields J(4-->5)=-65 cm(-1).