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

[Hongsheng Zhao]

On 07/28/2011 04:38 PM, Phillip Nyawere wrote:
> To implement no symmetry use
> nosym=.true. at the $system

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
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>
>
>     § Gabriele Sclauzero, EPFL SB ITP CSEA
>     /   PH H2 462, Station 3, CH-1015 Lausanne/
>
>
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> --
> Phillip W. Otieno Nyawere,
> The Abdus Salam International Center for Theoretical Physics,
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-- 
Hongsheng Zhao <zhaohscas at yahoo.com.cn>
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China