[Pw_forum] Unit cell problem

[mohsen modaresi]

Dear users,
I work on ZnS, I faced with a problem. When i use a unit cell with 2 atom
every things is OK and the bandstructure has a very good agreement with
previous calculation but when i use bigger unit cell every things go wrong,
Where is the problem of my calculation?
I set my input files for a unit cell with 2 & 16 atoms at below.

&SYSTEM
                       ibrav = 2,
                   celldm(1) =10.221527872,
                      nbnd = 50,
                         nat = 2,
                        ntyp = 2,
                     ecutwfc = 40 ,
                     ecutrho = 160 ,
                 occupations = 'fixed' ,
 /
 &ELECTRONS
                  conv_thr = 0.0000001 ,
                  diago_full_acc = .TRUE.,
 /
ATOMIC_SPECIES
   Zn    65.409   zn_pz.upf
   S     32.066   s_pz.upf
ATOMIC_POSITIONS {angstrom}
 Zn     0.0   0.0   0.0
 S      1.352  1.352  1.352
K_POINTS {automatic}
6 6 6   0  0  0



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

 &SYSTEM
                       ibrav = 2,
                   celldm(1) =20.446835196 ,
                      nbnd = 90,
                         nat = 16,
                        ntyp = 2,
                     ecutwfc = 40 ,
                     ecutrho = 160 ,
                 occupations = 'fixed' ,
 /
 &ELECTRONS
                  conv_thr = 0.0000001 ,
                  diago_full_acc = .TRUE.,
 /
ATOMIC_SPECIES
   Zn    65.409   zn_pz.upf
   S     32.066    s_pz.upf
ATOMIC_POSITIONS {angstrom}
 Zn     0.0   0.0   0.0
 Zn     2.705   2.705   0.0
 Zn     2.705   0.0   2.705
 Zn     5.409   2.705   2.705
 Zn     0.0   2.705   2.705
 Zn     2.705   5.409   2.705
 Zn     2.705   2.705   5.409
 Zn     5.409   5.409   5.409
 S      1.352   1.352   1.352
S      4.057   4.057   1.352
S      4.057   1.352   4.057
S      6.762   4.057   4.057
S      1.352   4.057   4.057
S      4.057   6.762   4.057
S      4.057   4.057   6.762
S      6.762   6.762   6.762
K_POINTS {automatic}
5 5 5   0  0  0



Thanks for your answers

Mohsen Modarsei

Ferdowsi University, Iran.
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