[Pw_forum] Optical phonon convergence at Gamma-point

[Axel Kohlmeyer]

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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-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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