[Pw_forum] SPIN

[Giovanni Cantele]

the starting magnetisation is provided as input variable, as detailed here:
http://www.quantum-espresso.org/wp-content/uploads/Doc/INPUT_PW.html#idm6316208 <http://www.quantum-espresso.org/wp-content/uploads/Doc/INPUT_PW.html#idm6316208>
and is needed to break the symmetry and allow pw.x to search for a magnetic solution with
energy lower that the non magnetic one, if it exists. So, even in absence of spin-orbit coupling,
if you want to study a magnetic ground state of a certain material, you have to provide it in input
to trigger the spin-polarised calculation.

The total magnetisation is instead used to impose a specific magnetisation on your system:
http://www.quantum-espresso.org/wp-content/uploads/Doc/INPUT_PW.html#idm6318192 <http://www.quantum-espresso.org/wp-content/uploads/Doc/INPUT_PW.html#idm6318192>
instead of allowing pw.x to determine it during the self-consistent cycle. Of course, if you
impose something it is not guaranteed that you are studying the lowest energy configuration.

Giovanni

> On 17 Nov 2016, at 14:24, ashkan shekaari <shekaari at gmail.com> wrote:
> 
> Dear Giovanni Cantele,
> 
> Thank you so much for the detailed response. As my last question, what is the exact difference between starting magnetization and total magnetization?
> 
> --
> Regards,
> Ashkan Shekaari
> Plasma Physics Research Center
> Science and Research Branch
> I A U, 14778-93855 Tehran, Iran.
> 
> On Thu, Nov 17, 2016 at 2:16 PM, Giovanni Cantele <giovanni.cantele at spin.cnr.it <mailto:giovanni.cantele at spin.cnr.it>> wrote:
> No, it is not necessary if you do not expect that the system under study has a magnetisation.
> See here:
> http://www.quantum-espresso.org/wp-content/uploads/Doc/pw_user_guide/node10.html#SECTION00043030000000000000 <http://www.quantum-espresso.org/wp-content/uploads/Doc/pw_user_guide/node10.html#SECTION00043030000000000000>
> 
> In practice, for solids where NO internal or external magnetic field is not present,
> time-reversal symmetry applies. This has the consequence that
> 
> E(k,up) = E(-k,down)
> 
> so for a non-magnetic solid, even in the presence of spin-orbit coupling, it is possible that the spin-up and spin-down
> densities are the same and the total magnetisation zero. Non-magnetic solids then can divided into
> systems with (spatial) inversion symmetry and systems where this symmetry does not apply.
> 
> The former have the additional constraint that E(k, any_spin)=E(-k,any_spin) that, combined with the previous equation,
> produces double SPIN degenerate bands (so each state AT GIVEN is at least double degenerate). Germanium is an example.
> 
> The latter show a lifting of this double SPIN degeneracy, like in the case of GaAs. The spin degeneracy is recovered ONLY
> for those k-points for which k and -k are equivalent (thus differing by only a reciprocal lattice vector). Gamma point is an example.
> 
> Giovanni
> 
> 
>> On 16 Nov 2016, at 22:17, ashkan shekaari <shekaari at gmail.com <mailto:shekaari at gmail.com>> wrote:
>> 
>> Dear experts,
>> 
>> Is starting magnetization necessary to have a full spin-orbit coupling?
>> 
>> What value should it take on?
>> --
>> Regards,
>> Ashkan Shekaari
>> Plasma Physics Research Center
>> Science and Research Branch
>> I A U, 14778-93855 Tehran, Iran.
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> -- 
> 
> Giovanni Cantele, PhD
> CNR-SPIN
> c/o Dipartimento di Fisica
> Universita' di Napoli "Federico II"
> Complesso Universitario M. S. Angelo - Ed. 6
> Via Cintia, I-80126, Napoli, Italy
> e-mail: giovanni.cantele at spin.cnr.it <mailto:giovanni.cantele at spin.cnr.it>
> Phone: +39 081 676910
> Skype contact: giocan74
> 
> ResearcherID: http://www.researcherid.com/rid/A-1951-2009 <http://www.researcherid.com/rid/A-1951-2009>
> Web page: http://people.na.infn.it/~cantele <http://people.na.infn.it/~cantele>
> 
> 
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-- 

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: http://people.na.infn.it/~cantele

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