# [Pw_forum] Problem in understanding the Phonon vibrational modes at gamma point

barnali bhattacharya tintinsonapakhi at gmail.com
Thu Oct 26 08:55:56 CEST 2017

```Dear Quantum ESPRESSO users,

I am trying to calculate the phonon vibrational modes at gamma point of a
periodic system by using quantum espresso. I successfully obtained the Mode
symmetry, VIBRATIONAL MODES and irreducible representations (GIVEN BELOW).
..........................................................................................
Mode symmetry, C_2v (mm2)  point group:

freq (  1 -  1) =          3.8  [cm-1]   --> A_1  D_1  S_1   I+R
freq (  2 -  2) =          6.9  [cm-1]   --> B_1  D_3  S_3   I+R
freq (  3 -  3) =         10.7  [cm-1]   --> B_2  D_4  S_4   I+R
freq (  4 -  4) =        158.2  [cm-1]   --> B_2  D_4  S_4   I+R
freq (  5 -  5) =        183.6  [cm-1]   --> A_2  D_2  S_2   R
----------------------------------------------------------------------------

------------------------------------------------------------------------------
freq ( 34 - 34) =       2000.0  [cm-1]   --> A_1  D_1  S_1   I+R
freq ( 35 - 35) =       2188.1  [cm-1]   --> A_1  D_1  S_1   I+R
freq ( 36 - 36) =       2213.4  [cm-1]   --> B_1  D_3  S_3   I+R
...............................................................................................................
>From the output, I have understood the following points-

The point group symmetry of my system is  C2v (mm2) and the number of
symmetry elements is 4 (A_1, A_2, B_1, and B_2)

There are 36 irreducible representations of the phonon modes (3 acoustic
and 33 optical modes) and all the modes are non-degenerate.

Optical phonon modes at the Γ-point are given by 11 A_1 +7 B_2+4 A_2+11B_1,

But I did not understand the 2nd last and 3rd last columns (D_1  S_1).
............D_4, S_4 ?

Sincerely
Barnali Bhattacharya
Ph.D scholar, Assam University,
Silchar-788011
Assam, INDIA
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