[Pw_forum] Pw_forum Digest, Vol 55, Issue 4

[Jun Liu]

Dear Dr. Baroni and other forum members,

Thanks for your kind and patient response on my question. I am glad to know
that there is a way to cheat QE to do a free electron gas calculation by
giving a null pseudo-potential. I guess this "null" means replacing every
number in the pp file with zero. If the case, then I might ask a bit
further on my question.

With a free electron gas system, plane wave expansion would be enough. But
what if I want to replace the plane wave basis with the plane wave + some
local orbital basis and do the calculation. Although a weird request, this
should be able to be done in principle. But the issue is how to introduce
the local orbits into the problem. Can I do something like this? Choose
some pp giving the local orbitals to be included in the calculation, and
set in the input pp file the local/nonlocal potential and the pseudo/AE
core charge to vanish, to mimic the fact that there is no ion existing in
the system. Then what is left in the pp file might render QE to do some
type of plane wave+local orbital calculation. Please let me know whether
this idea works and whether these modifications are sufficient for the
purpose.

I might here ask a very fundamental question. The reason we choose the
sophisticated PAW treatment by introducing projector/pseudo/AE wavefunction
directly relies on the non-smoothness nature of the valence orbital itself,
but not on a specific ion to be treated. That is to say, we don't need to
know what is behind the scene when deciding whether PAW treatment is needed
given a specific valence orbital in the pp file. Am I right?

Thanks,

Sincerely,
Jun Liu



> Message: 6
> Date: Wed, 4 Jan 2012 09:16:34 +0100
> From: Stefano Baroni <baroni at sissa.it>
> Subject: Re: [Pw_forum] how to treat free electron gas, especially how
>        to setup input file
> To: PWSCF Forum <pw_forum at pwscf.org>
> Message-ID: <18E5109A-639A-43DC-9FBE-FC8308CFEF79 at sissa.it>
> Content-Type: text/plain; charset="iso-8859-1"
>
> Hem, what a weird request!
> Why not doing the exercise with pencil-and-paper?
> If you insist using a very sophisticated code to do that, you have two
> options, I believe, both of which will require somewhat more work than the
> simple pencil-and-paper exercise.
> dig into the code where the the hamiltonian*wavefunctions operations are
> performed (the routine used to be called "h_psi", and simply have the the
> multiplication times the external potential skipped, which will set the
> electrons "free" ...
> build a fake pseudo-potential which is simply null, and feed it into the
> computer ...
> HTH - Stefano B
>
>
> On Jan 3, 2012, at 6:42 PM, Jun Liu wrote:
>
> > Dear All,
> >
> > Let me wish all of you a Happy New Year first.
> >
> > Maybe this is a very fundamental problem, but I cannot google any hint
> online, so here I am. I would like to work out a free electron gas problem
> with QE. I know total number of charges, and the size of the box the
> electrons move in and probably periodic b.c. applied to it. Now how can I
> possibly set up the input file for it? It seems many fundamentional
> properties of a lattice do not come with free electron gas. The code itself
> should be well-defined for a lattice system, so how is the code adapted to
> the free electron problem?
> >
> > Thank you very much,
> >
> > Sincerely,
> > Jun Liu
> >
> >
> >
> > _______________________________________________
> > Pw_forum mailing list
> > Pw_forum at pwscf.org
> > http://www.democritos.it/mailman/listinfo/pw_forum
>
> ---
> Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
> http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) /
> stefanobaroni (skype)
>
> La morale est une logique de l'action comme la logique est une morale de
> la pens?e - Jean Piaget
>
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-- 
Sincerely,
Jun
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