[Pw_forum] Questions on supercell calculation of MgB2

[张 洪彬]

Dear All:
  Now I am trying to do some calculation about properties of MgB2 using 
supercell method, however, there are some problems which I don't 
understand: I have done a calculation to check the convergence of supercell 
method by calculating a 3*3*1 supercell without atom loss, but the band 
energy is very different with those of original unit cell, where parameters 
are the same except nat=27,nband=108, results are shown below: for /gamma 
points only
      k = 0.0000 0.0000 0.0000     band energies (ev):

   -12.0174 -10.2019 -10.2018 -10.2018 -10.2018 -10.2018 -10.2018  -7.2039
    -7.2039  -7.2039  -7.2039  -4.7605  -4.7605  -4.7605  -4.7605  -4.7605
    -4.7605  -4.7511  -4.7511  -1.1770  -1.1769  -1.1769  -1.1769  -1.1769
    -1.1769   0.7865   0.7865   4.2776   4.2776   4.2776   4.2776   4.2776
     4.2776   4.6061   5.5116   6.7205   6.7205   6.7205   6.7206   6.7206
     6.7206   7.4459   7.4459   7.4459   7.4459   7.4999   7.4999   9.9779
     9.9779   9.9779   9.9779   9.9779   9.9779  11.0717  11.0717  11.1852
    13.0561  13.0561  13.0561  13.0561  13.0561  13.0561  13.4153  13.4153
    13.4153  13.4153  13.4153  13.4153  14.4582  14.4582  14.4582  14.4582
    14.9112  16.4510  16.4510  16.5120  16.5120  16.5120  16.5120  17.2677
    17.2677  17.2677  17.2677  17.2677  17.2677  18.5590  18.5590  18.5590
    18.5591  18.5591  18.5591  19.5087  19.5087  19.5730  19.5730  19.5730
    19.5730  19.5730  19.5731  20.6149  20.6149  20.6149  20.6149  20.6149
    20.6149  21.1872  24.7356  24.7356
while for orignal unit cell, those energy value are:
            k = 0.0000 0.0000 0.0000     band energies (ev):

    -4.9799   4.4905   8.0545   8.0548   9.3576  13.7607  13.7610  15.5855
    17.5768  20.6496  25.7276  26.3962

Further more, when I want to plot the band structure using bands.x to get 
the .dat file, there is an error like:
 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

     from punch_band : error #         5
      increase maxdeg
 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


     stopping ...

And at the beggining of the scf calculation procedure, there are 
"additional transition found" in the output window of Pwgui, does this 
means the space group has been changed? Since I didn't omit any atom in the 
structure, why or why not?
Thank you very much for any comments.

Hongbin Zhang

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