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When to use plus/minus displacements to generate Hellmann-Feynman forces?

Generally, the usage of plus/minus displacements, while generatng Hellmann-Feynman forces, leads to higher accuracy. This is specially true for low symmetry crystals where the atoms reside in sites with site symmetry which does not have symmetry plane. If the site symmetry has a symmetry plane, such that the applied displacement is perpendicular to this plane, then calculation for a single plus displacement is sufficient.