Dear Sir,

I'd like to ask spin-orbit coupling in VASP.

The manual indiates the use of "SAXIS and MAGMOM" for this.

Two approaches are suggested as follows.

MAGMOM = x y z ! local magnetic moment in x,y,z

SAXIS = 0 0 1 ! quantisation axis parallel to z

or

MAGMOM = 0 0 total_magnetic_moment ! local magnetic moment parallel to SAXIS

SAXIS = x y z ! quantisation axis parallel to vector (x,y,z)

1. Which approach is recommended?

If my system use 3 atoms, should I enter 9 values for MAGMOM?

2. How can I know and input the values of x y z for MAGMOM in the first approach? What's the physical backgound to guess x y z?

3. Generally, may I use the default values of MAGMOM (i.e., without determination of MAGMOM) and SAXIS 0 0 1 for spin-orbit coupling?

if it is difficult to get initial guess of MAGMOM, what input values should be recommended (e.g., random values between certain minimum and maximum)?

4. For instance, for SnSe materials (symmetry: pnma and/or cmcm, two atoms in primitive cell),

what values of MAGMOM (for SAXES 0 0 1) should I use respectively (for pnma and cmcm)?

Thank you very much for your answers.

Best regards,

Young