Spin-orbit coupling breaks symmetry of forces in HSE calculation

[hipertrofia]

Hi all,

I am performing a HSE06 calculation with spin-orbit coupling for a zinc-blende material. The unit cell is perfectly symmetric, and given by the lattice vectors (in units of the lattice parameter):

0 1/2 1/2
1/2 0 1/2
1/2 1/2 0

The asymmetric unit is given by

0 0 0 <- atom A
1/4 1/4 1/4 <- atom B

either in a "direct lattice" representation in the fashion of VASP, or in units of the lattice parameter in Cartesian coordinates.

The strange thing that I do not understand is that after the relaxation atom B is moved by about 0.003 Angstrom in the (Cartesian) direction (-1,-1,+1). I allow atom B to move freely, not because I expect it to do so, but because I didn't bother changing my T T T tags to F F F, knowing by experience (and intuition) that in a ZB lattice without shear deformation atom B should stay put.

Is this due to the spin-orbit coupling?

Is it a spurious result due to having switched off symmetry in the calculation (ISYM = -1)?

Why does this not happen in an LDA calculation?

I might be naive here, but any help would be greatly appreciated.

Thanks,
Miguel

[support_vasp]

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