[Pw_forum] NRAPP not giving phonon frequencies

[Bipul Rakshit]

Dear Lorenzo,
Thanks for your reply.
One more thing I want to ask is that, after computing first 3 modes using
nrapp, where do i find the dynamical matrix? Because in LaMnO3.dyn2 file is
empty, after the calculation is finished.

Is the non-diagonalised matrix is printed elsewhere?

regards

On Tue, Dec 1, 2009 at 1:14 PM, Lorenzo Paulatto <paulatto at sissa.it> wrote:

> Quoting Bipul Rakshit <bipulrr at gmail.com>:
> > The calculation is finished with the following lines
> >
> >   *   End of self-consistent calculation
> >      Convergence has been achieved
> >      Stopping because representation    4 is not done*
> >
> > But in LaMnO3.dyn2 or in the output file, no where it is giving me
> > frequencies.
> > So can anybody tell me how to get the frequencies of only first three
> modes,
> > rather than calculating all 60 modes?
> > What I have to modify in my input file?
>
>
> Dear Bipul,
> it is not possible to compute the frequency of a single mode; each
> mode gives you a line of the dynamical matrix; the matrix is then
> diagonalized and each eigenvalue correspond to a frequency. If you
> only compute the modes corresponding to the first atom you can still
> compute the frequency belonging to the first atom ***but*** under the
> approximation that all the other atoms have infinite masses; i.e. a
> mass much larger than the first one.
>
> This clearly is **not** your case, as Lanthanium is 2.5 times heavier
> than Manganese and almost 9 times heavier than Oxygen. Nevertheless,
> I'll give you the procedure as reference, but don't be tempted to use
> it in your case, unless you really know what you're doing.
>
> After computing the first 3 mode you have the first 3 lines of the
> dynamical matrix. You can take the 3x3 minor of the the matrix (first
> 3 rows and first 3 columns) and diagonalize it by hand (using octave,
> matlab or whatever software you prefer). The eigenvalues are the
> frequencies you are looking for in the said approximation. They are in
> Rydberg atomic units, you can get cm-1 multiplying by 109737.
>
> best regards.
>
>
> --
> Lorenzo Paulatto (MS)
> SISSA & Democritos
>
>
>
>
>
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>
>
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-- 
Dr. Bipul Rakshit
Research Associate,
S N Bose Centre for Basic Sciences,
Salt Lake,
Kolkata 700 098
India
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