  An approach to compute exchange parameters of the Heisenberg model in
plane-wave-based methods is presented. This calculation scheme is based on the
Green's function method and Wannier function projection technique. It was
implemented in the framework of the pseudopotential method and tested on such
materials as NiO, FeO, Li2MnO3, and KCuF3. The obtained exchange constants are
in a good agreement with both the total energy calculations and experimental
estimations for NiO and KCuF3. In the case of FeO our calculations explain the
pressure dependence of the N\'eel temperature. Li2MnO3 turns out to be a Slater
insulator with antiferromagnetic nearest-neighbor exchange defined by the spin
splitting. The proposed approach provides a unique way to analyze magnetic
interactions, since it allows one to calculate orbital contributions to the
total exchange coupling and study the mechanism of the exchange coupling.
