[Pw_forum] how can I create a supercell in PWSCF?

[Lorenzo Paulatto]

In data 24 aprile 2009 alle ore 16:51:31, Tone Kokalj <tone.kokalj at ijs.si>  
ha scritto:
> To make it right, make sure you switch to the co-called "translational
> asymmetruc unit" display (click second left image at the bottom
> toolbox). Then you save an XSF file. This file will contain a lot of
> info. Search for ATOMS section. These are atoms you need!

this is nice, I have used it a couple of time myself, it has a couple of  
minor drawback though that you have to be aware of:
1. the new atomic positions will be in angstrom units
2. you have to redefine the unit cell by hand (the one defined in the  
PRIMVEC section of the XSF file is in Angstrom, not rescaled to fit the  
supercell)

In the CVS version of QE there is a tentative and experimental support for  
aritmetic expression, with this in mind you can define a supercell easily  
if you are using alat units in this way:

1. use ibrav=0 and specify the cell axis by hand, leaving celldm(0) (aka  
alat) unchanged, e.g. if you are doing a 2x2x1 FCC you will have:
   CELL (alat)
      -1   0    1
       0   1    1
      -0.5 0.5  0

2. you can then copy/past the atomic position four times (in units of  
alat) and change them by adding one each time, i.e. let's suppose you have  
silicon:
Si  0     0     0
Si 1/4   1/4   1/4
Si  1     0     0
Si 1/4+1  0     0
...
Si 1/4+1 1/4+1  0

A major drwaback is that you have to do it by hand/script and that the  
resulting input file is not readable by xrysden.

best regards

-- 
Lorenzo Paulatto
SISSA  &  DEMOCRITOS (Trieste)
phone: +39 040 3787 511
skype: paulatz
www:   http://people.sissa.it/~paulatto/

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