# [Pw_forum] Phonon branch enumeration

e kb eminekb at yahoo.com
Tue Mar 10 08:57:30 CET 2009

```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_2gSo 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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