[Pw_forum] Body Centered Tetragonal

[Gabriele Sclauzero]

Dear Trinh

vtmtrinh at caltech.edu wrote:
> Hi Eyvarz,
> 
> 
> Thanks for your message.
> Yes, I did this way before.  However, when I checked the output, the
> crystal axes (cart. coord. in units of a_0) are still the same ones as
> defined in the input_PW, i.e:
> 
> V1 = a/2(1,-1,c/a), V2=a/2(1,1,c/a), V3=a/2(-1,-1,c/a)

Yes, the crystal axis are fixed once you've chosen the lattice (i.e. ibrav), the 
bohr,alat,crystal,angstrom options only specify how to input the atomic coordinates, that 
are subsequently converted to the internal format.

Isn't it easy to build a transformation matrix from your choice for the crystal axis to 
the convention followed in pwscf, rather than modify the code?

> 
> If use these vectors to duplicate the structure e.g in z-direction (make
> supercell to see how it looks like), the duplicated structure lie in wrong
> direction (as I doubted).  This makes sense since the original structure
> has the crystal axes as:
> 
> p1=(a,0,0), p2=(0,a,0), and p3=(a/2, a/2, c/2).

In order to duplicate the structure in the z-direction (Cartesian), in my view the easiest 
choice is to use a simple tetragonal cell (ibrav=6), which has the 3rd axis oriented along z.

> 
> I think another way to have the consistency is to project the original
> structure to the crystal axes used in QE .  Then use the resulting
> structure for the input_PW. 

Yes this is the way I was suggesting earlier, if I understand well... seems the more 
convenient to me. You simply need to find the transformation matrix from one frame of 
reference to the other.


>(I thought by simply changing the primitive
> vectors in the code would be faster if I knew where it is  :-)).  

Maybe faster, but not safer unless you know all possible side effects.

Good luck,


GS

>Let me
> try this way and see how it goes.
> 
> Best,
> 
> Trinh
> 
> 
> 
> 
> 
> 
>> Hi,
>>
>> --- On Wed, 1/27/10, vtmtrinh at caltech.edu <vtmtrinh at caltech.edu> wrote:
>>
>>> Therefore, I cannot use the current coordinates that I have
>>> for the input to run pw.x.
>> You can use "angstrom" keyword in Atomic_Positions if you have them in
>> Cartesian and Angstrom, or "alat" - again, Cartesian, but wrt the lattice
>> parameter a_0.
>>
>> Bests,
>> Eyvaz.
>>
>> -------------------------------------------------------------------
>> Prof. Eyvaz Isaev,
>> Theoretical Physics Department, Moscow State Institute of Steel & Alloys,
>> Russia,
>> Department of Physics, Chemistry, and Biology (IFM), Linkoping University,
>> Sweden
>> Condensed Matter Theory Group, Uppsala University, Sweden
>> Eyvaz.Isaev at fysik.uu.se, isaev at ifm.liu.se, eyvaz_isaev at yahoo.com
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
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> 
> 
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