[Pw_forum] Optical phonon convergence at Gamma-point

[Sophia Nishad]

Thank you, Prof. Baroni, and Alex.

Sophia
UCB, USA.

On Mon, May 25, 2009 at 10:41 AM, Axel Kohlmeyer <
akohlmey at cmm.chem.upenn.edu> wrote:

> On Mon, 2009-05-25 at 06:21 -0700, Sophia Nishad wrote:
> >
> > Thanks, Paolo. I have little confusion about your comment though.
> >
> > Earlier I had used norm-conserving PP in supercell calculations as
> > well; that produced converged frequency of  518.2 for Si. So, is it
> > possible that linear response calculations might not give better
> > results compared to supercell calculations (which is very expensive).
> > The result could be sensitive to the assumptions of linear response
> > theory as well??
>
> no. there also is the error induced by using the electron density and
> the potential on a finite grid. those are somewhat random, depending
> on choice of k-points and atom positions and availability of FFT grids.
> if you get less than 2% deviation from experiment then this is rather
> by chance than by constructions.
>
> cheers,
>   axel.
>
> >
> > Thanks,
> > Sophia
> >
> >
> > On Mon, May 25, 2009 at 1:26 AM, Paolo Giannozzi
> > <giannozz at democritos.it> wrote:
> >
> >         On May 24, 2009, at 23:04 , Sophia Nishad wrote:
> >
> >         > it is still off from the experiemntal value by ~8cm-1.
> >
> >
> >         both pseudopotentials and approximations to the
> >         exchange-correlation
> >         functional introduce intrinsic errors. It is quite hard to get
> >         any
> >         better
> >         results than those you get.
> >
> >         Paolo
> >         ---
> >         Paolo Giannozzi, Democritos and University of Udine, Italy
> >
> >
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> >
> >
> >
> >
> > --
> > Sophia
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>
> --
> =======================================================================
> Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
>   Center for Molecular Modeling   --   University of Pennsylvania
> Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
> tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
> =======================================================================
> If you make something idiot-proof, the universe creates a better idiot.
>
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-- 
Sophia
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