In the presence of a 2D crystal and spin-orbit coupling, the SU(2) spin axial vector (Sx,Sy,Sz) decouples into an in-plane (Sx,Sy) and out-of-plane part Sz. For Quantum ESPRESSO, is there a way to turn on only the out-of-plane component of spin-orbit coupling?
I happen to know that the effect of the in-plane spin-orbit is marginal for the bands near the Fermi level (as observed by photo-emission experiments). Therefore, I imagine turning off the in-plane part would not only be computationally easier, but also more accurate to my material.
I don't see why this shouldn't be possible, seeing that it amounts to a constraint that the up/down spin should be conserved (i.e not mixed at the level of the Hamiltonian). I believe we would say the spins should remain "collinear". However, I can't seem to find anything on this; and when I choose the following parameters
lspinorb = .true. noncolin = .false.
then Quantum ESPRESSO (pw.x) immediately crashes with the following complaint (written in CRASH file):
spin orbit requires a non collinear calculation
Compared to a non-spin-orbit scf calculation, the only other thing I'm doing differently is use fully relativistic pseudopotentials:
Any insight would be greatly appreciated!