[Pw_forum] Non cubic structures

[Srijan Kumar Saha]

   Dear Users,

   I think there is one error in the letter of Miguel MartÃnez Canales.
   He wrote :

   ibrav= 8, celldm(1)=a, celldm(2)=b/a, celldm(3)=c/a,

> nat = 20 , ntyp = 3



     I think the corrected one should be :

    ibrav= 8, celldm(1)=a (here 'a' is in Bohr Unit (alat)),

    celldm(2)=b/a (but here 'a' should be in Angstrom unit)

    Am I right Miguel??????????



> Dear Users  I'm also having a simple confusion.


   My question:


> ATOMIC_POSITIONS {bohr}
> >  Y    0.0000000E+00   0.0000000E+00   0.0000000E+00
> >  Y     7.1680498   7.1680498   7.1680498
> >  Cr1   3.5840249   3.5840249   3.5840249
> >  Cr2  10.7520752  10.7520752  10.7520752
> >  O     3.5840249   0.0000000E+00   3.5840249
> >  O     3.5840249   3.5840249   0.0000000E+00
> >  O     0.0000000E+00   3.5840249   3.5840249
> >  O     7.1680498   3.5840249   3.5840249
> >  O     3.5840249   7.1680498   3.5840249
> >  O     3.5840249   3.5840249    7.1680498
>



>
> >
> >
> >  If I  use 2 formula unit then  it should be
> >
> > Y    0.0000000E+00   0.0000000E+00   0.0000000E+00
> >  Y     7.1680498   7.1680498   7.1680498
> >
> >
>



>  OR just
>



> Y    0.0000000E+00   0.0000000E+00   0.0000000E+00
> >  Y     7.1680498        0.0000000E+00   0.0000000E+00
> >
> >
> >shifted by 0.5 in X-direction only.
>


   I'm also very much confused with  the construction of correct super cell.

  My main confusion is if my system is non cubic (like ibrav=8) and I
generate
  a 2 unit  supercell  then what I'll  put  for  ibrav?????
 Will I give the same ibrav=8?????
 What'll be  the  value of celldm(1),  celldm(2), celldm(3)??????
 It's really strange that there are so many simple example in the pwscf
example directory
 but not a single example discussing  that type of  confusion.

>  Looking forward to ur soonest reply.
>

   Thanks in Advance.

   Srijan
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