  The calculation of interatomic magnetic exchange interactions entering the
Heisenberg model from the standpoint of the density functional theory (DFT) is
outlined for two Fe-based molecular magnets: a trinuclear complex with a Schiff
base ligand, which makes an antiferromagnetically coupled frustrated system,
and a model bipyrimidine-connected planar network of Fe ions. First-principles
electronic structure calculations are performed using the real-space method
Siesta and the full-potential linearized augmented plane wave FLAPW method
FLEUR, correspondingly. We discuss the application of fixed spin moment
technique for preparing the system in a given magnetic configuration, and the
effect of intraatomic Coulomb correlation, approximated by the LDA+U technique,
on the values of interaction parameters.
