[Pw_forum] Is maxirr=N working in phonon calculations?

[Stefano Baroni]

PS: randomness is necessary to make sure (almost sure ...) that  
degenerate eigenvectors belong to an irredeucible (rather than  
reducible) representation. Stefano

On Nov 6, 2008, at 8:25 AM, Stefano Baroni wrote:

>
> On Nov 6, 2008, at 2:03 AM, Serge Nakhmanson wrote:
>
>> Dear Paolo,
>>
>> Thank you very much for exhaustively answering my questions, even
>> the one that I did not ask, i.e., if irreps are picked at random,
>> then what is the purpose of using "maxirr=N"? :))
>>
>> Your comment on randomness of irrep generation makes sense to me.
>> I only know a general answer to how it is done -- by induction
>> from an (Abelian) group of crystal translations -- but am
>> rusty on particular numerical algorithms. However, I got an
>> excellent new book by El-Batanouny and Wooten (ISBN 9780521828451),
>> all the details should be in there.
>
> I do not know about the book, but I am afraid that what is  
> implemented in the code is not anything worth reporting in a book.  
> The idea is as follows. The code already contains all the weaponery  
> for simmetrizing all kind of stuff with respect to the small group  
> of the point in the Brillouin zone we are working at. In particular,  
> you fill the dynamical matrix with random numbers and then  
> symmetrize it. The resulting matrix will commute with all the  
> operations of the small group, and its eigenvectors will therefore  
> realize a number of irreducible representation of the group. As  
> simple as that. Do not ask me about the details of the  
> implementation, though. The documentation (as poor as it may be) is  
> the code itself ...
>
> Cheers - SB
>
>>
>>
>> THX again,
>>
>> S.
>>
>> Paolo Giannozzi wrote:
>>> Serge Nakhmanson wrote:
>>>
>>>> Still the code blasted through all 10 irreps.
>>>
>>> not sure whether the "maxirr" option really works. In the CVS
>>> version it was replaced by other options that are useful for
>>> Grid execution (i.e. splitting a long phonon calculation in
>>> many smaller pieces rto be run on different machines). Its main
>>> usage was to prevent the code from crashing for cpu time limit
>>> in a ungraceful way, but option "max_seconds" can do the same.
>>>
>>>> Also, is there any system to which irreps are computed first and
>>>> which later? Or is this process completely random, i.e. it would
>>>> be futile to cover a certain frequency window by computing only
>>>> some, but not all, irreps?
>>>
>>> I don't think this is going to work. Not sure how exactly
>>> irreps are selected. If no symmetry is present, taht's easy:
>>> atom 1 component x, y,z, atom 2 component x, y, z and so on.
>>> In presence of symmetry, the irreps are obtained via a procedure
>>> in which random matrices are symmetrized, so the order in which
>>> they are produced is quite unpredictable (also the accuracy of
>>> this answer is unpredictable, since it is based on vague
>>> recollections)
>>>
>>> Paolo
>>
>>
>> -- 
>> *********************************************************
>>  Serge M. Nakhmanson               phone: (630) 252-5205
>>  Assistant Scientist                 fax: (630) 252-4798
>>  MSD-212, Rm. C-224
>>  Argonne National Laboratory
>>  9700 S. Cass Ave.
>>  Argonne, IL 60439
>> *********************************************************
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>
> ---
> Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
> Trieste
> http://www.sissa.it/~baroni / [+39] 040 3787 406 (tel) -528 (fax) /  
> stefanobaroni (skype)
>
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> morale de la pensée - Jean Piaget
>
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
http://www.sissa.it/~baroni / [+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 pensée - Jean Piaget

Please, if possible, don't  send me MS Word or PowerPoint attachments
Why? See:  http://www.gnu.org/philosophy/no-word-attachments.html






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