# [Pw_forum] Phonon branch enumeration

Jeffrey Mullen jtmullen at ncsu.edu
Tue Mar 10 12:13:58 CET 2009

```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.

Cheers
Jeff

e kb wrote:

> Dear Jeff,
> I am definitely not an expert on the topic but I'd like to understand
> your question better.
>
>     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
>
>_______________________________________________
>Pw_forum mailing list
>Pw_forum at pwscf.org
>http://www.democritos.it/mailman/listinfo/pw_forum
>
>
>
>------------------------------------------------------------------------
>
>_______________________________________________
>Pw_forum mailing list
>Pw_forum at pwscf.org
>http://www.democritos.it/mailman/listinfo/pw_forum
>
>

```