[Pw_forum] questions about symmetry and the elphon output

[Jia-An Yan]

Hi everyone,

I am calculating the el-ph interactions of graphene with Pwscf 3.1.1. There
are two questions here:

1) First thing is about the symmetry obtained with pw.x. For the same
graphene system, when I use ibrav=0 with cell parameters, I got only 8
symmetry opertions( with inversion). There are six modes in the phonon
spectra. However, since the graphene system is hexagonal, I rotate the
coordinate system and use ibrav=4 to define the system, then I got 24 sym
operations (with inversion). There are also six modes (with 2 degenerate
modes) obtained. Why does the pw.x obtained different symmetry for the same
system? Can I adjust it manually? Here is my input:

C.scf.fit.in:

 &control
    calculation='scf'
    restart_mode='from_scratch',
    prefix='C',
    pseudo_dir = '/dsgpfs/jayan/celph',
    outdir='/dsgpfs/jayan/scratch',
    wf_collect=.true.
 /
 &system
    ibrav=  0, celldm(1) =4.628,nat=2,ntyp= 1,
    ecutwfc =30.0,nr1=24,nr2=24,
    occupations='smearing', smearing='methfessel-paxton', degauss=0.02,
    la2F=.true.,
 /
 &electrons
    conv_thr =  1.0d-10
    mixing_beta = 0.7
 /
ATOMIC_SPECIES
 C  12.0107  C_nc
ATOMIC_POSITIONS (alat)
C  0.0000000000    0.0000000000     0.0000000000
C  0.5773502693    0.0000000000     0.0000000000
K_POINTS {automatic}
 20 20 1 0 0 0
CELL_PARAMETERS
  0.866025404   -0.500000000    0.000000000
  0.000000000    1.000000000    0.000000000
  0.000000000    0.000000000    6.000000000


2) The second question is about the output of the ph.x. I notice that the
C.elph.out output following lines (2 2 1 1):

     There are   4 irreducible representations

     Representation     1      2 modes - To be done

     Representation     2      2 modes - To be done

     Representation     3      1 modes - To be done

     Representation     4      1 modes - To be done
     PHONON       :    16.94s CPU time

but at the end of the calculation, I got the omega as follows:

     omega( 1) =      16.131001 [THz] =     538.075842 [cm-1]
     omega( 2) =      16.131001 [THz] =     538.075842 [cm-1]
     omega( 3) =      30.116408 [THz] =    1004.581911 [cm-1]
     omega( 4) =      36.922309 [THz] =    1231.603836 [cm-1]
     omega( 5) =      36.922309 [THz] =    1231.603836 [cm-1]
     omega( 6) =      39.058597 [THz] =    1302.863199 [cm-1]

Note that the order of the omega is different from the calculation order (2
1 2 1). Then I am quite confused at this, why are the output omega not the
same order as the real calculation? Did the progam sort them after
completion? I am interested in some data for each representation, so I need
to get the omega for the corresponding representation.

Thanks for your help in advance!

Jia-An Yan
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