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<LI><A NAME="tex2html1369"
  HREF="node65.html#SECTION000138010000000000000">12.8.0.1  Is there a simple way to determine the symmetry of a given 
phonon mode?</A>
<LI><A NAME="tex2html1370"
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<H2><A NAME="SECTION000138000000000000000">
12.8  Phonons </A>
</H2>

<P>

<H4><A NAME="SECTION000138010000000000000">
12.8.0.1  Is there a simple way to determine the symmetry of a given 
phonon mode?</A>
</H4> 

<P>
A symmetry analyzer was added in v.3.2 by Andrea Dal Corso. 
Other packages that perform symmetry analysis of phonons and normal modes:
<BR>
ISOTROPY package: http://stokes.byu.edu/iso/isotropy.html
<BR>
ACKJ, ACMI packages: http://www.cpc.cs.qub.ac.uk.

<P>

<H4><A NAME="SECTION000138020000000000000">
12.8.0.2 I am not getting zero acoustic mode frequencies, why? </A>
</H4>

<P>
Because the Acoustic Sum Rule (ASR), i.e. the translational invariance,
is violated in approximated calculations. In PW calculations, 
the main and most irreducible violation comes from the discreteness
of the FFT grid. There may be other reasons, though, notably
insufficient convergence: "Recently I found that the parameters
<TT>tr2_ph</TT> for the phonons and <TT>conv_thr</TT> for the 
ground state can affect the quality of the phonon calculation,
 especially the "vanishing" frequencies for molecules."
(Info from Katalyn Gaal-Nagy). Anyway: if the nonzero frequencies are
small, you can impose the ASR to the dynamical matrix, usually with
excellent results.

<P>
Nonzero frequencies for rotational modes of a molecule are a fictitious 
effect of the finite supercell size, or else, of a less than perfect
convergence of the geometry of the molecule.

<P>

<H4><A NAME="SECTION000138030000000000000">
12.8.0.3 Why do I get negative phonon frequencies? </A>
</H4>

<P>
"Negative" frequencies actually are "imaginary" frequencies 
(<!-- MATH
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 -->
<IMG
 WIDTH="25" HEIGHT="39" ALIGN="MIDDLE" BORDER="0"
 SRC="img34.png"
 ALT="$ \omega^{2}_{}$"> &lt; 0). If these occur for acoustic frequencies at Gamma point, 
or for rotational modes of a molecule, see above.
In all other cases: it depends. It may be a problem of bad
convergence (see above), or it may signal a real instability.

<P>

<H4><A NAME="SECTION000138040000000000000">
12.8.0.4 Why do I get a message <EM>no elec. field with metals</EM>? </A>
</H4>

<P>
If you want to calculate the contribution of macroscopic electric 
fields to phonons - a quantity that is well-defined in insulators 
only -- you cannot use smearing in the scf calculation, or else the
code will complain. 

<P>

<H4><A NAME="SECTION000138050000000000000">
12.8.0.5 How can I calculate Raman/IR coefficients in metals?</A>
</H4>

<P>
You cannot: they are well defined only for insulators.

<P>

<H4><A NAME="SECTION000138060000000000000">
12.8.0.6 How can I calculate the electron-phonon coefficients
in insulators?</A>
</H4>

<P>
You cannot: the current implementation is for metals only.

<P>
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