[Pw_forum] my understanding about ASR

[Stefano Baroni]

Soft modes at small (but finite) q can indeed be an indication of an  
elastic or a mechanical instability. At q=0, there should be exactly  
thre zero modes, though, irrespective of the occurrence of such an  
instability (I believe). Imaginary frequencies can also occur at q=0,  
but _in addition_ to the three zero modes mentioned above. cheers - SB

On Jun 19, 2008, at 7:10 PM, stewart at cnf.cornell.edu wrote:

> Hi Stefano,
>
> You raise a good point and perhaps I should have been more careful  
> in my
> previous statement.  I agree that based on symmetry, the total  
> forces on the
> system should be zero and there should be a set of zero mode.   
> Applying the
> acoustic sum rule reinstates this fact.  From my practical  
> experience, I
> recall having greater problems with negative frequencies at the  
> Gamma point
> for systems that had the lattice constant greater than the  
> equilibrium value
> (expanded system).  This occurred for direct calculations at small q  
> and
> also based on interpolating the phonon dispersion from the q grid.   
> Here you
> get phonon softening which shifts the phonon dispersion curves down  
> and
> perhaps this leads to problems with calculating the phonon  
> dispersion from
> the given q grid near Gamma.  I took this as an indication of the
> instability of the expanded structure, but perhaps this is not the  
> best way
> to view it.
>
> Best regards,
>
> Derek
>
>
> Stefano Baroni writes:
>
>>
>> On Jun 19, 2008, at 4:17 PM, stewart at cnf.cornell.edu wrote:
>>
>>> Fig 2 (D. A. Stewart, New. J. Phys. 10 043025 (2008)).  Of course,  
>>> the
>>> difference you get between ASR results and direct calculations will
>>> depend
>>> strongly on how well relaxed your structure is.
>>
>> hmmm: this may well be the case, but if this is so, it must be for a
>> nontrivial reason (which, if true, would qualify the "of course" as
>> slightly misplaced).
>>
>> the total force acting on any isolated system is of course zero, even
>> though the system is not at equilibrium ("not well relaxed"). in
>> particular, this total force is invariant ("invariantly zero!") with
>> respect to a homogeneous translation of the system. therefore, the   
>> system
>> must have a zero mode (actually three of them: one for each   
>> cartesian
>> coordinate), irrespective on how well (or how bad) the  internal  
>> degrees
>> of freedom have been optimized.
>>
>> is this wrong?
>>
>> SB
>>
>>>
>>>
>>> http://dx.doi.org/10.1088/1367-2630/10/4/043025
>>>
>>> Best regards,
>>>
>>> Derek
>>>
>>>
>>> Paolo Giannozzi writes:
>>>
>>>> xwzhang at uestc.edu.cn wrote:
>>>>
>>>>> are there any problem that the slope of the phonon around Gamma is
>>>>> incorrect, because ASR cure the problem of k=0, but what about  
>>>>> other
>>>>> points near it?
>>>>
>>>> for phonons at k/=0, the error due to ASR violation seems to be
>>>> usually negligible. I never tried to make calculations with
>>>> very small q, though.
>>>>
>>>> Paolo
>>>> -- 
>>>> Paolo Giannozzi, Democritos and University of Udine, Italy
>>>> _______________________________________________
>>>> Pw_forum mailing list
>>>> Pw_forum at pwscf.org
>>>> http://www.democritos.it/mailman/listinfo/pw_forum
>>>>
>>>
>>>
>>>
>>> ################################
>>> Derek Stewart, Ph. D.
>>> Scientific Computation Associate
>>> http://www.people.cornell.edu/pages/das248/
>>> 250 Duffield Hall
>>> Cornell Nanoscale Facility (CNF)
>>> Ithaca, NY 14853
>>> stewart (at) cnf.cornell.edu
>>> (607) 255-2856
>>> _______________________________________________
>>> 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
>> [+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
>>
>>
>>
>>
>
>
>
> ################################
> Derek Stewart, Ph. D.
> Scientific Computation Associate
> 250 Duffield Hall
> Cornell Nanoscale Facility (CNF)
> Ithaca, NY 14853
> stewart (at) cnf.cornell.edu
> (607) 255-2856
> _______________________________________________
> 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://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






-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20080619/aa95c74e/attachment.html>