We created 2 × 1 × 2 supercells from the primitive cells to allow either configurations with either FM or AFM ordering along each of the three principal directions: M-Cl-M, Morganic-M, and M interlayer. We then calculated the energies of the eight possible magnetic configuration from these supercells and fitted their magnetic superexchange interactions to the magnetic Hamiltonian (Eqn. 3, Tab. 3). We found that the superexchange was very sensitive to a U , with too small U producing unphysically large exchange.