# [Pw_forum] Phonon branch enumeration

Dal Corso Andrea dalcorso at sissa.it
Tue Mar 10 14:53:27 CET 2009

```On Tue, 2009-03-10 at 07:13 -0400, Jeffrey Mullen wrote:
> Emine,
>
> I am interested in extracting the matrix elements of the interaction
> <k+q| dV | k> from elphon.f90, which is simple enough to do, however,
> the question has come up that we do not know from this portion of the
> code what the order of the modes are in this array in a general way. For
> instance, I cannot see (from the code) if omega(2-3) is Representation 1
> or Representation 4.
>

It is an interesting idea. I think it is possible to make the
symmetry classification of the displacements patterns with the same
routine that makes the symmetry classification of the eigenvectors
of the dynamical matrix. Presently, this is not done. You can
code this feature yourself or just use iverbosity=1. With this option
the code writes the displacement patterns and you can
check their symmetry.

Andrea

>
> Cheers
> Jeff
>
>
>
> e kb wrote:
>
> > Dear Jeff,
> > I am definitely not an expert on the topic but I'd like to understand
> >
> >     Mode symmetry, D_6h(6/mmm) point group:
> >
> >     omega(  1 -  1) =        -37.4  [cm-1]   --> A_2u            I
> >     omega(  2 -  3) =        -33.9  [cm-1]   --> E_1u            I
> >     omega(  4 -  4) =        863.3  [cm-1]   --> B_2g
> >     omega(  5 -  6) =       1336.0  [cm-1]   --> E_2g
> >
> > So you have the irr. representations and the values for omega (values
> > are printed with higher precision in the previous part after
> > diagonalization as you reported). What exactly do you want to know
> > more (that you cannot check from a book on group theory) ?
> >
> > Emine Kucukbenli, grad. student, SISSA, Italy.
> >
> > --- On *Tue, 3/10/09, Jeffrey Mullen /<jtmullen at ncsu.edu>/* wrote:
> >
> >     From: Jeffrey Mullen <jtmullen at ncsu.edu>
> >     Subject: [Pw_forum] Phonon branch enumeration
> >     To: pw_forum at pwscf.org
> >     Date: Tuesday, March 10, 2009, 2:34 AM
> >
> >Greetings
> >
> >I am testing electron-phonon interaction calculations with graphene and
> >have encountered a problem enumerating the resulting phonon branches.
> >When I run the ph.x calculation at the q(0 0 0), the code calculates 4
> >representations:
> >
> >     Representation #  1 modes #   1  2
> >     Representation #  2 mode #   3
> >     Representation #  3 mode #   4
> >     Representation #  4 modes #   5  6
> >
> >
> >When the values for omega are calculated, the results are:
> >
> > **************************************************************************
> >     omega( 1) =      -1.119851 [THz] =     -37.354451 [cm-1]
> >     omega( 2) =      -1.016557 [THz] =     -33.908905
> > [cm-1]
> >     omega( 3) =      -1.016557 [THz] =     -33.908905 [cm-1]
> >     omega( 4) =      25.882237 [THz] =     863.344237 [cm-1]
> >     omega( 5) =      40.051667 [THz] =    1335.988701 [cm-1]
> >     omega( 6) =      40.051667 [THz] =    1335.988701 [cm-1]
> > **************************************************************************
> >
> >
> >with the following mode symmetries:
> >
> >     Mode symmetry, D_6h(6/mmm) point group:
> >
> >     omega(  1 -  1) =        -37.4  [cm-1]   --> A_2u            I
> >     omega(  2 -  3) =        -33.9  [cm-1]   --> E_1u            I
> >     omega(  4 -  4) =        863.3  [cm-1]   --> B_2g
> >     omega(  5 -  6) =       1336.0  [cm-1]   --> E_2g            R
> >
> >
> >My question is one of ordering. How do I extract which omega(##)
> >corresponds to which representation/mode?
> >
> >Thanks
> >Jeff Mullen
> >North Carolina State
> > University
> >
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> >
> >
> >
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--
Andrea Dal Corso                    Tel. 0039-040-3787428
SISSA, Via Beirut 2/4               Fax. 0039-040-3787528
34014 Trieste (Italy)               e-mail: dalcorso at sissa.it

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