[Pw_forum] problem with nbnd

[vicky singh]

Hi
i am studying trying to relax the structure of Ni - Al system super cell
with 40 atoms. what i understood till now is that if we don't specify nbnd
it takes the defalt value. To test this i performed relax calculation with
pure Ni with 1 atom first with out specifing any nbnd value then with nbnd =
10.  the results were 6.66 equilibrium latt_parameter -2.62 kbar pressure
and fermi energy 14.1765 for default nbnd and 6.66 equilibrium
latt_parameter -2.5 kbar pressure and fermi energy 14.1765 for nbnd =10.
there is slight change in the pressure value. why?

similar calculation i performed for the larger structure of Ni - Al system
super cell with 40 atoms input file attached. but there is significant
variation in the results. I have considered nbnd = 195 because i have 35 Ni
and rest Al atoms.

*condition  (1)*
*INPUT*
&SYSTEM
nspin = 2,  starting_magnetization(1)=0.7
nbnd = 195
*OUTPUT*
          total   stress  (Ry/bohr**3)                   (kbar)     P=
3.59
   0.00002296   0.00000000   0.00000000          3.38      0.00      0.00
   0.00000000   0.00002296   0.00000000          0.00      3.38      0.00
   0.00000000   0.00000000   0.00002739          0.00      0.00      4.03

     the Fermi energy is    13.5287 ev
!    total energy              =   -3027.65131607 Ry
     Harris-Foulkes estimate   =   -3027.65131529 Ry
     estimated scf accuracy    <       0.00000010 Ry

     The total energy is the sum of the following terms:

     one-electron contribution =   -1562.85025157 Ry
     hartree contribution      =    1281.18429645 Ry
     xc contribution           =   -1063.08426181 Ry
     ewald contribution        =   -1682.87860135 Ry
     smearing contrib. (-TS)   =      -0.02249780 Ry

     total magnetization       =    16.07 Bohr mag/cell
     absolute magnetization    =    19.19 Bohr mag/cell

ATOMIC_POSITIONS (crystal)
Ni       0.000000000   0.000000000   0.003462687
Ni       0.500000000   0.500000000   0.003077321
Ni       0.500000000   0.000000000   0.052355136
Ni       0.000000000   0.500000000   0.052355136
Ni       0.000000000   0.000000000   0.101475754
Ni       0.500000000   0.500000000   0.101664051
Ni       0.500000000   0.000000000   0.150808385
Ni       0.000000000   0.500000000   0.150808385
Ni       0.000000000   0.000000000   0.200000000
Ni       0.500000000   0.500000000   0.200000000

*condition  (2)*
*INPUT*
&SYSTEM
nspin = 2,  starting_magnetization(1)=0.7

*OUTPUT*
         total   stress  (Ry/bohr**3)                   (kbar)     P=
2.93
   0.00001834   0.00000000   0.00000000          2.70      0.00      0.00
   0.00000000   0.00001834   0.00000000          0.00      2.70      0.00
   0.00000000   0.00000000   0.00002317          0.00      0.00      3.41
     the Fermi energy is    13.5289 ev

!    total energy              =   -3027.65133623 Ry
     Harris-Foulkes estimate   =   -3027.65132967 Ry
     estimated scf accuracy    <       0.00000007 Ry

     The total energy is the sum of the following terms:

     one-electron contribution =   -1577.38934683 Ry
     hartree contribution      =    1288.44897083 Ry
     xc contribution           =   -1063.08376215 Ry
     ewald contribution        =   -1675.60469092 Ry
     smearing contrib. (-TS)   =      -0.02250718 Ry

     total magnetization       =    16.06 Bohr mag/cell
     absolute magnetization    =    19.18 Bohr mag/cell
ATOMIC_POSITIONS (crystal)
Ni       0.000000000   0.000000000   0.003594618
Ni       0.500000000   0.500000000   0.003232516
Ni       0.500000000   0.000000000   0.052460864
Ni       0.000000000   0.500000000   0.052460864
Ni       0.000000000   0.000000000   0.101507073
Ni       0.500000000   0.500000000   0.101682285
Ni       0.500000000   0.000000000   0.150774629
Ni       0.000000000   0.500000000   0.150774629
Ni       0.000000000   0.000000000   0.200000000
Ni       0.500000000   0.500000000   0.200000000
Ni       0.500000000   0.000000000   0.249225371
Ni       0.000000000   0.500000000   0.249225371
Ni       0.000000000   0.000000000   0.298492927

there is significant variation in the stress, energy and the final
coordinate of the atoms. i am not able to understand why it is getting size
dependent. can anybody please help.

vicky singh

research student
Bangalore
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