When to use plus/minus displacements to generate HellmannFeynman forces? Generally, the usage of plus/minus displacements,
while generatng HellmannFeynman 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.
