[Pw_forum] the missing symmetry of bands

Stefano de Gironcoli degironc at sissa.it
Thu Apr 9 10:17:49 CEST 2009


程迎春 wrote:
> Actually the symmetry of 4H-AlN is C_6v, but the pw.x can not find all 
> the 12 operations. So I followed the user's guide Page 60, I changed 
> PW/eqvect.f90. The value of accep is changed to be 10^-3.  The changed 
> pw.x can find all 12 operations.  This change is the reason why you 
> got C_3v symmetry. 
>  
> After I rechanged the value of accep to be 10^-5, pw.x only can find 6 
> operations.  Can you give some additional advices to find other 
> operations ?
specify atomic coordinates with a larger number of (correct) digits..

Stefano de Gironcoli - SISSA and DEMOCRITOS


> 2009/4/8 Dal Corso Andrea <dalcorso at sissa.it <mailto:dalcorso at sissa.it>>
>
>     On Wed, 2009-04-08 at 19:34 +0800, 程迎春 wrote:
>     > Dear PWscf users,
>     >         In my recent calculation of 4H-AlN (ABCB stacking sequence),
>     > the band symmetry seems to be missed in bands.x calculation.  The
>     > output of bands.x is as following:
>     >
>      **************************************************************************
>     >                     xk=(   0.00000,   0.00000,   0.00000  )
>     >      double point group C_6v (6mm)
>     >      there are  9 classes and  3 irreducible representations
>     >      the character table:
>     >        E     -E     C2    2C3  -2C3   2C6  -2C6   3s_v  3s_d
>     >                    -C2                           -3s_v -3s_d
>     > G_7    2.00 -2.00  0.00  1.00 -1.00  1.73 -1.73  0.00  0.00
>     > G_8    2.00 -2.00  0.00  1.00 -1.00 -1.73  1.73  0.00  0.00
>     > G_9    2.00 -2.00  0.00 -2.00  2.00  0.00  0.00  0.00  0.00
>     >      the symmetry operations in each class:
>     >      E             1
>     >       C2  -C2      2   -2
>     >       2C6          3    4
>     >       2C3          5    6
>     >       3s_v-3s_v    7   -7    9   10  -10   -9
>     >       3s_d-3s_d    8   -8   12  -11   11  -12
>     >      -E           -1
>     >      -2C6         -3   -4
>     >      -2C3         -5   -6
>     >      Band symmetry, C_6v (6mm)  double point group:
>     >      e(  1 -  2) =     -8.01764  eV     2   -->   ?
>     >      e(  3 -  4) =     -7.17573  eV     2   -->   ?
>     >      e(  5 -  6) =     -7.13551  eV     2   -->   ?
>     >      e(  7 -  8) =     -5.93881  eV     2   -->   ?
>     >      e(  9 - 10) =      1.12077  eV     2   --> G_7
>     >      e(  9 - 10) =      1.12077  eV     2   -->   0 G_8
>     >      e( 11 - 12) =      3.70317  eV     2   -->   0 G_7
>     >      e( 11 - 12) =      3.70317  eV     2   --> G_8
>     >      e( 13 - 14) =      3.76782  eV     2   -->   ?
>     >      e( 15 - 16) =      6.34352  eV     2   -->   ?
>     >      e( 17 - 18) =      6.35869  eV     2   --> G_9
>     >      e( 19 - 20) =      6.58061  eV     2   -->   ?
>     >      e( 21 - 22) =      6.59473  eV     2   --> G_9
>     >      e( 23 - 24) =      6.65103  eV     2   -->   ?
>     >      e( 25 - 26) =      6.66500  eV     2   --> G_9
>     >      e( 27 - 28) =      6.91040  eV     2   -->   ?
>     >      e( 29 - 30) =      6.92400  eV     2   --> G_9
>     >      e( 31 - 32) =      7.00967  eV     2   --> G_7
>     >      e( 31 - 32) =      7.00967  eV     2   -->   0 G_8
>     >      e( 33 - 34) =     11.19288  eV     2   -->   ?
>     >      e( 35 - 36) =     13.31415  eV     2   -->   ?
>     >      e( 37 - 38) =     13.59769  eV     2   -->   ?
>     >      e( 39 - 40) =     14.02975  eV     2   -->   ?
>     >      e( 41 - 42) =     17.15974  eV     2   -->   ?
>     >      e( 43 - 44) =     17.32327  eV     2   -->   ?
>     >      e( 45 - 46) =     17.92003  eV     2   -->   ?
>     >
>      **************************************************************************
>     > My question are :
>     > 1. what does "?" mean in "  e(  1 -  2) =     -8.01764  eV     2
>
>     It means that the symmetry finder is confused. The bands have not a
>     symmetry that it recognizes.
>     There are several reasons for this:
>
>     1) The 'scf' and 'bands' calculations have been run with slightly
>     different parameters. (solution: recheck your input)
>
>     2) Your atomic positions are not sufficiently accurate: the number of
>     digits is sufficient for pw.x to find the symmetries but the
>     wavefunctions are sufficiently inaccurate to confuse the symmetry
>     analyzer. (solution: correct your atomic positions)
>
>     3) If 1) and 2) do not work and you are sure about your coordinates,
>     then probably there is a bug in the symmetry analyzer. (solution:
>     report
>     to the forum. I will try to solve it if I can).
>
>     In your particular case: I am confused. On my PC the symmetry
>     found with
>     your coordinates is C_3v and the symmetry analyzer seems to work. So
>     please recheck your run. If you still find C_6v please recheck your
>     atomic positions.
>
>     Hope this helps.
>
>     Andrea
>
>
>
>     >  -->   ?"      ?
>     > 2. what does "0" mean in "     e(  9 - 10) =      1.12077  eV     2
>     > -->   0 G_8" ?
>     > 3. In 2H-AlN(AB stacking sequence), all the symmetries of bands are
>     > shown. Why in 4H-AlN, it seems much more complicated.
>     >
>     > The input files about 4H-AlN for pw.x and bands.x are as following:
>     > scf.in <http://scf.in/>
>     > &control
>     >                     title = 2h_aln,
>     >                       prefix = '4h_band',
>     >                  calculation = 'scf' ,
>     >                 restart_mode = 'from_scratch' ,
>     >                   wf_collect = .false. ,
>     >                       outdir = './' ,
>     >                       wfcdir = './' ,
>     >                   pseudo_dir = './' ,
>     >                      tstress = .true. ,
>     >                      tprnfor = .true. ,
>     >                    wf_collect = .true. ,
>     >  /
>     >  &SYSTEM
>     >                        ibrav = 4,
>     >                    celldm(1) = 5.866002785691,
>     >                    celldm(3) = 3.238615289089,
>     >                          nat = 8,
>     >                         ntyp = 2,
>     >                      ecutwfc = 80,
>     >                        nbnd  = 46,
>     >                        nosym = .false. ,
>     >                      lspinorb = .true.,
>     >                      noncolin= .true.,
>     >                    starting_magnetization=0.00,
>     >  /
>     >  &ELECTRONS
>     >             electron_maxstep = 100,
>     >                     conv_thr = 1.D-10,
>     >  /
>     > ATOMIC_SPECIES
>     >     N   14.00700  N.rel-pbe-rrkjus.UPF
>     >    Al   26.98200  Al.rel-pbe-rrkj.UPF
>     > ATOMIC_POSITIONS (crystal)
>     > N        0.000000000   0.000000000  -0.000722968
>     > N        0.000000000   0.000000000   0.499277032
>     > N        0.333333333   0.666666667   0.249293699
>     > N        0.666666667   0.333333333   0.749293699
>     > Al       0.000000000   0.000000000   0.188800277
>     > Al       0.000000000   0.000000000   0.688800277
>     > Al       0.333333333   0.666666667   0.437593675
>     > Al       0.666666667   0.333333333   0.937793675
>     > K_POINTS automatic
>     >   8 8 8 1 1 1
>     >
>     > band.in <http://band.in/>
>     > &control
>     >                       title = 2h_aln,
>     >                       prefix = '4h_band',
>     >                  calculation = 'bands' ,
>     >                 restart_mode = 'from_scratch' ,
>     >                   wf_collect = .false. ,
>     >                       outdir = './' ,
>     >                       wfcdir = './' ,
>     >                   pseudo_dir = './' ,
>     >  /
>     >  &SYSTEM
>     >                        ibrav = 4,
>     >                    celldm(1) = 5.866002785691,
>     >                    celldm(3) = 3.238615289089,
>     >                          nat = 8,
>     >                         ntyp = 2,
>     >                      ecutwfc = 80,
>     >                      nbnd=46,
>     >                          lspinorb = .true.,
>     >                      noncolin= .true.,
>     >                    starting_magnetization=0.0,
>     >  /
>     >  &ELECTRONS
>     >             electron_maxstep = 100,
>     >                     conv_thr = 1.D-10,
>     >  /
>     > ATOMIC_SPECIES
>     >     N   14.00700  N.rel-pbe-rrkjus.UPF
>     >    Al   26.98200  Al.rel-pbe-rrkj.UPF
>     > ATOMIC_POSITIONS (crystal)
>     > N        0.000000000   0.000000000  -0.000722968
>     > N        0.000000000   0.000000000   0.499277032
>     > N        0.333333333   0.666666667   0.249293699
>     > N        0.666666667   0.333333333   0.749293699
>     > Al       0.000000000   0.000000000   0.188800277
>     > Al       0.000000000   0.000000000   0.688800277
>     > Al       0.333333333   0.666666667   0.437593675
>     > Al       0.666666667   0.333333333   0.937793675
>     > K_POINTS
>     >          2
>     >   0.000000  0.000000  0.000000  1.00
>     >   0.000000  0.000000  0.083333  1.00
>     >
>     >
>     > bands.in <http://bands.in/>
>     > &inputpp
>     > prefix='4h_band'
>     > outdir='./'
>     > filband='spin.dat'
>     > lsigma(3)=.true.
>     > lsym=.true.
>     > /
>     >
>     > Thank you in advance!
>     >
>     >
>     > --
>     > Y. C. Cheng
>     > Department of Phyics
>     > Nanjing University
>     > Nanjing 210093
>     > P. R. China
>     > Tel: 86-25-83592907
>     > Email: yccheng.nju at gmail.com <mailto:yccheng.nju at gmail.com>
>     >
>     > _______________________________________________
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>     > http://www.democritos.it/mailman/listinfo/pw_forum
>     --
>     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
>     <mailto:dalcorso at sissa.it>
>
>
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>
>
>
> -- 
> Y. C. Cheng
> Department of Phyics
> Nanjing University
> Nanjing 210093
> P. R. China
> Tel: 86-25-83592907
> Email: yccheng.nju at gmail.com <mailto:yccheng.nju at gmail.com>
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