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 <[email protected]> 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. > _______________________________________________ > Pw_forum mailing list > [email protected] > http://pwscf.org/mailman/listinfo/pw_forum -- 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: [email protected] 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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