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Turn on/off ISYM makes a big difference ~ 20kB in XY,YZ,ZX stress in deformed FCC Al Crystal structure.

What is more, only deformation in Z direction shows this problem. Any suggestion?

in general, it has to make a difference whether the forces and stress tensor components are symmetrized or not, depending on your input configuration, of course (if ISYM is set, the initial symmetry is preserved because the stress and forces are symmetrized with respect to the point group symmetry operations found for the initial geometry).