This system consists of one d 6 Fe 2+ center, one d 5 Fe 3+ center and a d 0 Al 3+ center. Since Al 3+ is closed shell, we do not consider any active space on it. We explore two different active spaces for this system as shown in Fig. , namely (11e,10o) with all 3d orbitals of both Fe atoms and (11e,20o) with all 3d and 4d orbitals of both Fe atoms. In these different parameters, we investigate the dependence of the J coupling according to the Yamaguchi