[Pw_forum] Defining spin states in PWscf

[Andrea Dal Corso]

On Fri, 2011-11-04 at 15:16 +0100, hariom jani wrote:
> Dear fellow QE users,
> 
> I am currently using fully relativistic pseudo-potentials to compute
> certain spin-orbit based properties in solids. 
> Given a silicon system, there are 4 valence and 4 conduction bands
> (each doubly degenerate in the spin components)  in total.
> 
> When i perform PWscf calculations, I set 'lspinorb' and 'noncolin' (in
> the system namelist) to .true. 
> 
> I desire that the 2 degenerate spin states of every band be defined as
> uniquely up and down (and not merely their linear combinations). 
> 
> 1. To achieve this I am currently using 'starting_spin_angle'=true,
> but is the correct command to use? If not, what should be done? 
> 2. Can this done, in spite of using the 'noncolin' command?
> 

It cannot be done.
When there is spin-orbit, the Hamiltonian does not commute with S_z so
you cannot divide the bands into up or down. If there is a strong
magnetization (much larger than spin-orbit) than the real bands do not
differ too much from up or down bands and it might be convenient to
start with wavefunctions that are much closer to spin-up and spin-down
states than to spin-angle states. In any case at self-consistency the
states are no more completely up or completely down. 

Andrea

> Thanks
> Hariom Jani
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-- 
Andrea Dal Corso                    Tel. 0039-040-3787428
SISSA, Via Bonomea 265              Fax. 0039-040-3787249
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