[Pw_forum] spin splitting

[vtmtrinh at caltech.edu]

Hi Gabriele,

Thanks for your reply. I am sorry for the unclear question.
The question is to calculate the splitting between the spin up and spin
down states in the conduction band (CB) of Si. For equlibrium Si bulk,
there should be no spin splitting. Therefore, the splitting can be induced
by moving the atoms a long phonon modes. The spin splitting is probably of
the order ~1e-7eV.

Yes, you are right.  I need to set nspin=2.  Since I also need to include
the spin-orbit coupling in this calculation, I have to use
noncolinear=.true.

I run a calculation yesterday, in which I set lspinorb=.true. and
noncolinear=.true. By setting this I think that spin-polarized calculation
was aslo done.  However, in the output, I don't see the eigen values for
spin up and spin down, but only one (like for the non-polarization case). 
I am not sure if I did not set the right parameters.  Attached is my
input.  Could you let me know what wrong with it.

Thanks,

MyTrinh

> 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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>
> --
>
>
> o ------------------------------------------------ o
> | Gabriele Sclauzero, PhD Student                  |
> | c/o:   SISSA & CNR-INFM Democritos,              |
> |        via Beirut 2-4, 34014 Trieste (Italy)     |
> | email: sclauzer at sissa.it                         |
> | phone: +39 040 3787 511                          |
> | skype: gurlonotturno                             |
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