[Pw_forum] About the Frozen Phonon Method

[Stefano Baroni]

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
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