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I'm trying to relax a supercell with defect in the center (ISIF = 2) in respect to forces EDIFFG =-1.0E-3. The atoms of horizontal boundary of the cell have been fixed and are only allowed to move in the boundary plane.

This configuration does not converge. I can see that forces on the 'bulk' atoms vanish but the forces on the fixed boundary do not. Please help me understand this behaviour and how to fix it.

atoms are not relaxed along the fixed directions, therefore the forces on them will not vanish for these components. they are not taken into account during the relaxation process and the check for convergence.