[Pw_forum] negative frequencies during phonon calculations

[C.H. Hu]

Dear P.M. Grant,

Thank you for your useful presentation. No, The system I consider is an
insulator.

Even tr2_ph was set to 1e-12, the negative frequency still can not be
eliminated. Maybe it must be further reduced as you have said. But it will
spend more computational time.

Regards,
C.H.

On 11/2/07, Paul M. Grant <w2agz at pacbell.net> wrote:
>
>  Dear C. H.,
>
>
>
> It would be helpful if we knew a little bit more about your system.  Is it
> a metal?  How many irreducible representations?
>
>
>
> The Eliashberg formalism for e-p interactions in metals involves a "double
> delta" integration over the Fermi energy dispersion at k-whatever, depends
> on the broadening (screening) parameter chosen, and converges
> excruciatingly…repeat painfully…slow.  As an example, see the following page
> from my web site (http://www.w2agz.com/snxaps07.htm) which contains the
> APS March Meeting talk I gave earlier this year on my (feeble) attempt to
> compute the gamma-point coupling in monoclinic (24 IRs !), 300 millikelvin
> superconductor, polysulfur nitride, (SN)x.  I used a tr2_ph = 1e-11, and
> you'll see the lowest two values of omega are negative (open my talk).  The
> computation took a day on a 4-year old dual Xeon box.  I figure a "serious"
> calculation would require tr2_ph = 1e-14 at least, or a computer on the
> scale of a Blue Gene.
>
>
>
> Sidebar for the ph.x developers:  It would be neat to be able to set a
> "suspension" mid-execution flag to be able to resume a long run later (maybe
> even on another computer).
>
>
>
> Having said all this, Quantum-Espresso is the only free package I know
> that even attempts to calculate alpha-squared F(omega).  Viva Italia.
>
>
>
> Regards, -Paul
>
> **
>

-- 
============================
C.H. Hu
Postdoctoral fellow
Chimie et Physico-Chimie appliquees
Institut Francais du Petrole (IFP)
Rueil-Malmaison, France
E_mail: chaohao.mse at gmail.com
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