[Pw_forum] About the Frozen Phonon Method

[mohaddeseh abbasnejad]

Dear Prof. Baroni

Thanks for your reply.
I wonder shouldn't the plot show the same frequency for a range of
displacements? And the same freq. for the range to be the harmonic
frequency? or the zero displacement limit, is the harmonic frequency?
 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. Shouldn't the plot have symmetry vs. the vertical
axis?

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
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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
Cellphone: +98 917 731 7514
E-Mail:     m.abbasnejad at gmail.com
Website:  http://physics.ut.ac.ir

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