Thank you, Prof. Baroni, and Alex.<br><br>Sophia<br>UCB, USA.<br><br><div class="gmail_quote">On Mon, May 25, 2009 at 10:41 AM, Axel Kohlmeyer <span dir="ltr"><<a href="mailto:akohlmey@cmm.chem.upenn.edu">akohlmey@cmm.chem.upenn.edu</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"><div class="im">On Mon, 2009-05-25 at 06:21 -0700, Sophia Nishad wrote:<br>
><br>
> Thanks, Paolo. I have little confusion about your comment though.<br>
><br>
> Earlier I had used norm-conserving PP in supercell calculations as<br>
> well; that produced converged frequency of 518.2 for Si. So, is it<br>
> possible that linear response calculations might not give better<br>
> results compared to supercell calculations (which is very expensive).<br>
> The result could be sensitive to the assumptions of linear response<br>
> theory as well??<br>
<br>
</div>no. there also is the error induced by using the electron density and<br>
the potential on a finite grid. those are somewhat random, depending<br>
on choice of k-points and atom positions and availability of FFT grids.<br>
if you get less than 2% deviation from experiment then this is rather<br>
by chance than by constructions.<br>
<br>
cheers,<br>
axel.<br>
<div class="im"><br>
><br>
> Thanks,<br>
> Sophia<br>
><br>
><br>
> On Mon, May 25, 2009 at 1:26 AM, Paolo Giannozzi<br>
> <<a href="mailto:giannozz@democritos.it">giannozz@democritos.it</a>> wrote:<br>
><br>
> On May 24, 2009, at 23:04 , Sophia Nishad wrote:<br>
><br>
> > it is still off from the experiemntal value by ~8cm-1.<br>
><br>
><br>
> both pseudopotentials and approximations to the<br>
> exchange-correlation<br>
> functional introduce intrinsic errors. It is quite hard to get<br>
> any<br>
> better<br>
> results than those you get.<br>
><br>
> Paolo<br>
> ---<br>
> Paolo Giannozzi, Democritos and University of Udine, Italy<br>
><br>
><br>
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><br>
><br>
><br>
> --<br>
> Sophia<br>
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--<br>
</div>=======================================================================<br>
<font color="#888888">Axel Kohlmeyer <a href="mailto:akohlmey@cmm.chem.upenn.edu">akohlmey@cmm.chem.upenn.edu</a> <a href="http://www.cmm.upenn.edu" target="_blank">http://www.cmm.upenn.edu</a><br>
Center for Molecular Modeling -- University of Pennsylvania<br>
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323<br>
tel: 1-215-898-1582, fax: 1-215-573-6233, office-tel: 1-215-898-5425<br>
=======================================================================<br>
If you make something idiot-proof, the universe creates a better idiot.<br>
</font><div><div></div><div class="h5"><br>
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</div></div></blockquote></div><br><br clear="all"><br>-- <br>Sophia<br>