[Pw_forum] spin splitting

[Gabriele Sclauzero]

Dear user,

vtmtrinh at caltech.edu wrote:
> Dear PWSCF Users,
> 
> Does any of you know how to calculate spin splitting using QE?  More
> clearly, if I have 2 structures (Si 8-atom cell), one at equilibrium and
> the other one with atoms being displced according to phonon mode.  I would
> like calculate the energies of the spin up and down.  There are 2 concerns

You did not clarify much, in my poor understanding. What do you want to calculate? The 
splittings between the spin up/spin down eigenvalues?
That should be easy, you do a calculation with nspin=2 and a guess for the starting 
magnetization and, if your system stays magnetic, you can easily compute the splittings 
from the list of eigenvalues given on output at the end of the calculation.

Or maybe you want to compute total energy difference between an AF and a FM configuration?

Why do you mention two configurations that differ by atomic displacements and ask about 
computing spin up/spin down splittings? Does magnetism arise in the second case due to the 
displacement from equilibrium configuration?


> that I have: (i) The difference in eneries for these 2 states is too
> small.  Can we get to that level of accuracy by using QE.  

It depends what is the accuracy you need. In principle you can increase it by increasing 
PW/charge-density cutoffs, number of k-points, and decreasing degauss (you may also need 
to decrease conv_thr or other things that did not come in my mind in this moment...). Then 
you probably  have to face with the error due to the use of pseudo-potentials, but 
generally the error in energy differences can be believed to be lower than absolute error 
on the total energies.

(ii) how to do
> it (how to fix spin up and down).

I don't understand exactly what you want to fix. You can for instance fix the total spin 
magnetization of your system using the tot_magnetization or multiplicity variables.

HTH

GS


> 
> Any suggestion would be appreciated.
> 
> Thank you,
> 
> MyTrinh Vo
> 
> Computation group
> JPL/CalTech
> 
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