[Pw_forum] FM and AFM calculations

Matteo Cococcioni matteo at umn.edu
Sun Aug 15 22:41:24 CEST 2010

Dear Padmaja,

starting_magnetization =/= 0 is used by th code to break the symmetry
between spin up and spin down and to "realize" there is
a magnetic solution of some kind. Of course, if you treat as magnetiic a
system that is not and you also provide a non zero
starting_magnetization, the code should still converge to a ground state
with zero magnetization (both total and absolute) i.e. with an equal number
of up and don electrons. Try with bulk Si.
In principle, once you give a finite, non zero magnetization on some atoms,
if the system has a magnetic ground state (either FM or AFM or any other
kind) the code should be able to find it. Some systems, however, have FM,
and various AFM local minima of the energy (metastable states) in which it
gets trapped depending on the magnetization pattern suggested in the input
through starting_magnetization. That's why it is sometimes important that
you try few different solutions.

the sign of magnetization doesn't matter. please keep in mind that in the
simplest LSDA or s-GGA functionals the spin of electrons is a scalar with
two possible values and no relationship with the lattice. if you want to
study more complex magnetization states
you have to use non-collinear functionals (where spin is a vector and
electronic wavefunction are spinors) and possibly
spin-orbit interactions.



On Sun, Aug 15, 2010 at 8:35 PM, Padmaja Patnaik <
padmaja_patnaik at yahoo.co.uk> wrote:

> Thank you for your explanation.
> Now if I want to do for FM calculations then i have to give some positive
> value of starting magnetization for all the elements even if they are not
> magnetic? For FM calculations can i give zero magnetization to few elements
> and positive value to other elements in the same compound?
> Regards
> Padmaja Patnaik
> Research Scholar
> Dept of Physics
> IIT Bombay
> Mumbai, India
> --- On *Sun, 15/8/10, pw_forum-request at pwscf.org <
> pw_forum-request at pwscf.org>* wrote:
> ----------------------------------------------------------------------
> Message: 1
> Date: Sat, 14 Aug 2010 16:39:26 +0200
> From: Matteo Cococcioni <matteo at umn.edu<http://mc/compose?to=matteo@umn.edu>
> >
> Subject: Re: [Pw_forum] FM and AFM calculations
> To: PWSCF Forum <pw_forum at pwscf.org<http://mc/compose?to=pw_forum@pwscf.org>
> >
> Message-ID:
>     <AANLkTimWOt3mc0S8nLBF6as=w-eW9+mxYje+-=3=S8Z4 at mail.gmail.com<http://mc/compose?to=S8Z4@mail.gmail.com>
> >
> Content-Type: text/plain; charset="iso-8859-1"
> Dear Padmaja,
> if you give zero starting_magnetization to all the atoms in your system you
> will obtain a non magnetic solution.
> The code won't be able to evolve any finite magnetization on any atom.
> what you should do is to give finite magnetization to atoms, some positive
> some negative, so that an antiferromagnetic
> solution can in principle appear. The AFM solution could be not unique,
> however, so different initial configurations
> could correspond to different final results.
> hope this helps,
> Matteo
> On Sat, Aug 14, 2010 at 4:02 PM, Padmaja Patnaik <
> padmaja_patnaik at yahoo.co.uk<http://mc/compose?to=padmaja_patnaik@yahoo.co.uk>>
> wrote:
> > Dear All
> >
> > I have a doubt in the input for FM and AFM calculations. I am calculating
> > for a compound semiconductor. If i have three different elements in the
> > sample. I want to proceed with FM calculations then in the input I give
> > nspin =2 and some positive value for starting magnetization for each
> element
> > in the sample. Again for AFM calculations I give the input as nspin =2
> and
> > zero starting magnetization for all the three elements. Is this correct?
> >
> > Thanking in advance
> > regards
> > Padmaja Patnaik
> > Research Scholar
> > Dept of Physics
> > IIT Bombay
> > Mumbai, India
> >
> > _______________________________________________
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