[Pw_forum] Can you help? (was "Fixed spin moment method & Phonon calculation")

[Nicola Marzari]

Dear Dr Ghosh,


I believe this is the current status - anyone please correct me:

1) you can only do phonons in magnetic systems (metallic or insulating)
using a "smearing" option in the SCF calculation - i.e. you cannot fix
the magnetization, but take whatever the system wants to be. You of 
course could try to do a standard SCF calculation imposing whatever 
magnetization you want, converge that, and restart it with no constraint 
and just smearing, and hope it remains where it is.

2) the NSCF calculation that follows can have or not have the smearing
(better test either). If your system is insulating, it should not matter
at all. If it is metallic/half-metallic, differences might arise (I 
would presume smearing in the NSCF would also be the right choice).

3) if the system is insulating, there is still something to do to fix
the effective charges, that at present are not correct. Hence, LO/TO
splitting at Gamma would be incorrect - everywhere else would be ok.

Subhradip Ghosh wrote:
> Dear Nicola,
> 
> A week back I had posted a question in the pw-forum. It is related to the
> feasibility of calculating phonons by fixing the magnetization. I had
> searched the archive for an answer but couldn't find one. Since I haven't
> gotten any reply from the forum, I am taking this liberty of disturbing
> you by forwarding the question. Can you please take some time from your
> schedule and send me a reply. The question is appended below.
> 
> Regards,
> 
> Subhradip
> 
> ---------------------------- Original Message ----------------------------
> Subject: [Pw_forum] Fixed spin moment method & Phonon calculation
> From:    "Subhradip Ghosh" <subhra at iitg.ernet.in>
> Date:    Wed, March 18, 2009 2:54 pm
> To:      pw_forum at pwscf.org
> --------------------------------------------------------------------------
> 
> I am interested in calculating phonn dispersions in a metallic system by
> fixin
> g the magnetization. To do that, I had fixed the magnetization to a
> particular value and tried the phonon calculation. However, it failed
> apprently because "the constrained magnetization is not implemented in the
> phonon code". Therefore, my question is:
>   Is phonon calculation not possible by fixing total magnetization with
> the present QE code?
> 
> I had searched the archive to find answers but no definite solution was
> found apart from a comment from Paolo which suggests that
> 
> "If the magnetization you choose is the ground state one, the two Fermi
> energies would be the same. In that case, commenting out the check in
> phq_readin.f90 should work. If you constrain the magnetization to a value
> that is not the ground state, I have no idea what numbers will come out".
> 
> Does this mean that a phonon calculation(by commenting out the check in
> phq_readin) by fixing the magnetization at a value other than the ground
> state value is not reliable? The answer to this question is important to
> me because I am interested in doing phonons by fixing total magnetization
> at values other than the ground state value.
> 
> Help in this regard would be useful.
> 
> Subhradip
> 
> ************************************************************************
>               Dr. Subhradip Ghosh
>               Assistant Professor
>               Department of Physics
>             Indian Institute of Technology
>                Guwahati,Assam-781039
>                     India
>                E-mail:subhra at iitg.ernet.in
>                Phone: +91 361 2582717(O)
>                       +91 361 2584717(R)
>                 Fax:  +91 361 2582749 (Physics)
>                       +91 361 2690762 (General)
> ************************************************************************
> 
> 
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Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
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