[Pw_forum] Using a big box model to calculating the energy of isolated atom

[Masoud Nahali]

Dear Hongsheng

In this case I think that the more important issue to be noticed is
considering spin polarization for most of atoms.  Many mistakes in this
issue arise from not considering spin polarization.
I guess Considering "Symmetry" may symmetrize the charge density and causes
some changes in electronic structure of atom (population of orbitals) or may
do reversely with nosym='true'.
Also, it is impossible for me to describe this issue based on DFT and the
exchange-correlation parts in details.

In quantum chemistry software based on Gaussian type orbitals basis set (not
plane wave basis set) we compare energy of an atom with considering
different spin multiplicities then find the lowest energy and call it
"ground state". I can do it for some atoms (whatever you select) by Firefly
software (former PC-GAMESS) and then you can compare it with the
corresponding calculation by QE.
These days I am very busy and going on a journey so let me do it in a due
course; contact me :)

Best Wishes

 Masoud



--------------
Masoud Nahali, Sharif University of Technology
masoud.nahali at gmail.com; a <http://alum.sharif.edu/%7Em_nahali>
lum.sharif.edu/~m_nahali

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Hongsheng Zhao Wrote :

 Thanks a lot for all of the helps on this topic.  I've checked and read
> most the topics on this ML which are related to "calculating the energy
> of isolated atom".  It seems this is  a tricky thing based on most of
> the replies on this similar topics on this ML.
>
> Based on my understanding, till now, there are still some issues
> puzzling me on this topic:
>
> 1- On the manual of pwscf (p37), we can find the following descriptions:
>
> --------
> ...
> Specify nosym=.true. to avoid generation of additional k-points in low
> symmetry
> ...
> -----------
>
> I want to know, by using nosym=.true., what's the different on the
> k-points set obtained with the ones without using this option?  For
> example, when using a bcc box with a atom at its center,  what's the
> corresponding   k-points set when using nosym=.true.?
>
> Furthermore, when we say a isolated atom, we mean a zero dimension
> system.  So the corresponding BZ  sampling by using gamma only should be
> OK.  If so,  there is no k-points issues at all - there only one k point
> should be considered.  Hence I'm confused on the description given by
> the manual *avoid generation of additional k-points in low symmetry*.
> Any hints?
>
> 2- From this ML, I found the following notes:
>
> ---------
> while doing calculations for any zero dimensions you should always set
> nosym =.true.
> -----------
>
> Does this mean: when I do the  calculations for isolated
> atom/molecule/nanocluster and so on, I should always set nosym =.true.?
>
> 3-  Prof. Paolo Giannozzi given the following descriptions on some
> similar topic in this ML:
>
> ---------
> calculations on isolated atoms with plane waves are tricky.
> Compare your results with what you get with the atomic code,
> in which you can fix occupancies at will
> ------------
>
> My issue: how to fix the occupancies based on the results?  Any examples?
>
> What do you mean by saying *at will*?  If really is a thing *at will*,
> why should we do it?
>
> Regards
>
> >
> > On Thu, Jul 28, 2011 at 10:12 AM, Gabriele Sclauzero
> > <gabriele.sclauzero at epfl.ch <mailto:gabriele.sclauzero at epfl.ch>> wrote:
> >
> >
> >     Il giorno 28/lug/2011, alle ore 01.55, Hongsheng Zhao ha scritto:
> >
> >>     Hi all,
> >>
> >>     I've learned that when using the a large-cell ( i.e., a big box ) to
> >>     calculate the energy of isolated atom, it should be done without
> >>     symmetry.   But I'm not sure what's the meaning of the "without
> >>     symmetry"  here and how to ensure this when we using a a big box
> >>     model
> >>     for this purpose.  Any hints on this issue?
> >
> >     This is a FAQ on this forum: have you spent some time browsing the
> >     archives?
> >
> >     OK, I'll give you a hint: "nosym"... but use it with care!
> >
> >     HTH
> >
> >     GS
> >
> >>
> >>     Regards
> >>     --
> >>     Hongsheng Zhao <zhaohscas at yahoo.com.cn
> >>     <mailto:zhaohscas at yahoo.com.cn>>
> >>     School of Physics and Electrical Information Science,
> >>     Ningxia University, Yinchuan 750021, China
> >>     _______________________________________________
> >>     Pw_forum mailing list
> >>     Pw_forum at pwscf.org <mailto:Pw_forum at pwscf.org>
> >>     http://www.democritos.it/mailman/listinfo/pw_forum
> >
> >
> >     ? Gabriele Sclauzero, EPFL SB ITP CSEA
> >     /   PH H2 462, Station 3, CH-1015 Lausanne/
> >
>
>
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