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

Hongsheng Zhao zhaohscas at yahoo.com.cn
Sat Jul 30 02:23:56 CEST 2011


On 07/30/2011 04:55 AM, Masoud Nahali wrote:
>
> 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)

When using plane wave basis set, say, pwcsf/vasp/castep and so on, what 
will happen in that case?

> we compare energy of an atom with considering
> different spin multiplicities then find the lowest energy and call it
> "ground state".

In plane-wave-basis-set based codes, we also can find the "ground 
state", what's the differences between these two "ground state" obtained 
by different  basis sets?

> 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.

I want to know: does this comparison make sense?

> These days I am very busy and going on a journey so let me do it in a
> due course; contact me :)

Thanks a lot for your kindly help and reply ;-)

>
> Best Wishes
>
>   Masoud
>
>
>
> --------------
> Masoud Nahali, Sharif University of Technology
> masoud.nahali at gmail.com <mailto:masoud.nahali at gmail.com>; a
> <http://alum.sharif.edu/%7Em_nahali>lum.sharif.edu/~m_nahali
> <http://lum.sharif.edu/~m_nahali>
>
> PSave a Tree . . . Please don't print this e-mail unless you really need to.
>
>
>
> 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>
>     <mailto: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>
>      >> <mailto:zhaohscas at yahoo.com.cn <mailto:zhaohscas at yahoo.com.cn>>>
>      >>     School of Physics and Electrical Information Science,
>      >>     Ningxia University, Yinchuan 750021, China
>      >>     _______________________________________________
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>      >> Pw_forum at pwscf.org <mailto:Pw_forum at pwscf.org>
>     <mailto: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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-- 
Hongsheng Zhao <zhaohscas at yahoo.com.cn>
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China


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