[Pw_forum] Problem with nscf calc. in elongated supercell

[Nicki Frank Hinsche]

Hi there,

I got some interesting problem troubling me for some time. I am going  
to calculate huge silicon supercells, therefore I am going step by  
step to larger supercells (lowering the symmetries) to check  
convergencies, before I am going to calculate the heavy perturbed  
system afterwards. I started with the diamond lattice (2 atoms, fcc) - 
 > no problem, perfect DOS and bandstructure. Then I lowered to bct  
(body centered tetragonal) with 4 atoms in the basis -> perfect,  
identical (normalized) DOS and perfect bandstructure. Then I switched  
to the cubic 8 atomic cell -> again everything excellent. Now I just  
doubled the cell in z-direction - so lets say two 8 atomic cells in a  
row -> and suddenly the DOS is awful, has no characteristics - nothing  
like before. The bands are nonsense. Despite the scf calculation went  
quite good (in case of wrong atomic positions for instance it would  
not be like that). I guess I am making a quite big mistake, but wonder  
myself whats wrong - I 've tried different k-meshs, smearings, cut- 
offs and I am quite sure its something systematic what I am doing  
wrong. Any idea? The Input files are attached....

thanks a lot, sorry for the long text

Nicki

SCF INPUT:

/
  &system
     ibrav= 0, celldm(1)= 10.266, nat= 16, ntyp= 1,
     ecutwfc =38.0, !smearing ='m-p', degauss =0.01
  /
  &electrons
     conv_thr =  1.0d-10
     mixing_beta = 0.25
  /
CELL_PARAMETERS {cubic}
1.0 0.0 0.0
0.0 1.0 0.0
0.0 0.0 2.0
ATOMIC_SPECIES
Si  28.086  Si.vbc.UPF
ATOMIC_POSITIONS
Si 0.00 0.00 0.00
Si 0.00 0.50 0.25
Si 0.50 0.00 0.25
Si 0.50 0.50 0.00
Si 0.25 0.25 0.125
Si 0.25 0.75 0.375
Si 0.75 0.25 0.375
Si 0.75 0.75 0.125
Si 0.00 0.00 0.50
Si 0.00 0.50 0.75
Si 0.50 0.00 0.75
Si 0.50 0.50 0.50
Si 0.25 0.25 0.625
Si 0.25 0.75 0.875
Si 0.75 0.25 0.875
Si 0.75 0.75 0.625
K_POINTS {automatic}
   12 12 6 0 0 0


NON SCF DOS CALCULATION:

  &system
     ibrav= 0, celldm(1)= 10.266, nat= 16, ntyp= 1,
     ecutwfc =38.0,nbnd=40, occupations='tetrahedra'
  /
  &electrons
     conv_thr =  1.0d-10
     mixing_beta = 0.25
  /
CELL_PARAMETERS {cubic}
1.0 0.0 0.0
0.0 1.0 0.0
0.0 0.0 2.0
ATOMIC_SPECIES
Si  28.086  Si.vbc.UPF
ATOMIC_POSITIONS
Si 0.00 0.00 0.00
Si 0.00 0.50 0.25
Si 0.50 0.00 0.25
Si 0.50 0.50 0.00
Si 0.25 0.25 0.125
Si 0.25 0.75 0.375
Si 0.75 0.25 0.375
Si 0.75 0.75 0.125
Si 0.00 0.00 0.50
Si 0.00 0.50 0.75
Si 0.50 0.00 0.75
Si 0.50 0.50 0.50
Si 0.25 0.25 0.625
Si 0.25 0.75 0.875
Si 0.75 0.25 0.875
Si 0.75 0.75 0.625
K_POINTS {automatic}
12 12 6 0 0 0


-------------------------------------------------------------
Nicki Frank Hinsche, Dipl. Phys.
Institute of physics - Theoretical physics,
Martin-Luther-University Halle-Wittenberg,
Von-Seckendorff-Platz 1, Room 1.07
D-06120 Halle/Saale, Germany
Tel.: ++49 345 5525460
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