Abstract
To investigate the effects of A-site cations on the magnetic interaction in highly distorted perovskite ACuO3 (A=Se, Te), we have set the microscopic Hamiltonian for the Cu–O–Cu–A cluster, taking into account the 
or 
hybridization between A-
or A-
and O-2p orbitals. We have found that (i) the superexchange interaction of the simple Cu–O–Cu triad without considering the A-site effect is inadequate to explain the ferromagnetism of SeCuO3, (ii) the 
hybridization weakens the superexchange interactions and (iii) the 
hybridization induces the ferromagnetic superexchange interaction along the c-axis. We have also verified that the 
hybridization affects the magnetic interaction through anisotropic charge transfer energy shift in the Cu–O–Cu interaction path.