[Pw_forum] About the Frozen Phonon Method

[Stefano Baroni]

On Aug 1, 2011, at 4:56 PM, mohaddeseh abbasnejad wrote:

> Dear Prof. Baroni
>  
> Thanks for your reply.
> I wonder shouldn't the plot show the same frequency for a range of displacements?

what range? why should it?

> And the same freq. for the range to be the harmonic frequency? or the zero displacement limit, is the harmonic frequency?

yes, by definition

> And when I test the displacements with the same amplitude, but different in sign (regarding the inward and outward atomic movements in the basis) I do not get the same results.

why should you?

> Shouldn't the plot have symmetry vs. the vertical axis?

why?

please, try you best to answer my questions, and I will make my best to let you understand

SB

>  
> Thank You again
> Mohaddeseh
> 
> 
> On Mon, Aug 1, 2011 at 6:33 PM, Stefano Baroni <baroni at sissa.it> wrote:
> Hi Mohaddesh. I do not understand why you say that the calculated frequency (which, I assume you define as 2 * Delta E / u^2 ???) does not converge as u->0. According to your plot, it does, and to a reasonable value. The finite slope of your plot around u=0 seems to me nothing but a manifestation of cubic anharmonicity ... What's wrong in this? SB
> 
> On Aug 1, 2011, at 3:01 PM, mohaddeseh abbasnejad wrote:
> 
>> 
>> Dear users,
>> 
>> I have plotted the harmonic frequency (obtained within the frozen phonon approach) of Diamond as a function of atomic displacements (small displacements in the [1 1 1] direction, u(1)=±(u,u,u) and u(2)=±(-u,-u,-u) are the displacements of two atoms in the basis). I expect to see a range of displacements that tend to the similar frequency, but there is no convergence in the plot. Also, the displacements with the same amplitude but different in sign, do not tend to the same frequency, which is not what I expect, too. The plot is attached. 
>> I was wondering if anyone could help me to find the source of error. I should expect the mentioned symmetry, right? Why there is no convergence?
>>  
>> Thanks in advance.
>>  
>> Yours,
>> Mohaddeseh
>> ---------------------------------------------------------
>> 
>> Mohaddeseh Abbasnejad, 
>> Room No. 323, Department of Physics, 
>> University of Tehran, North Karegar Ave.,
>> Tehran, P.O. Box: 14395-547- IRAN 
>> E-Mail:     m.abbasnejad at gmail.com
>> Website:  http://physics.ut.ac.ir
>> 
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> 
> ---
> Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
> http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)
> 
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> -- 
> ---------------------------------------------------------
> 
> Mohaddeseh Abbasnejad, 
> Room No. 323, Department of Physics, 
> University of Tehran, North Karegar Ave.,
> Tehran, P.O. Box: 14395-547- IRAN 
> Tel. No.: +98 21 6111 8634  & Fax No.: +98 21 8800 4781
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+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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