[Pw_forum] Error in User Guide - ibrav

[also at if.ufrj.br]

Dear Paolo,

The user guide contains the following basis triplet for the BCC structure:

bcc body entered cubic
====================
v1 = (a/2)(1,1,1),  v2 = (a/2)(-1,1,1),  v3 = (a/2)(-1,-1,1).


While the for BCT structure this is what is seen:


body centered tetragonal (i)
================================
   v1 = (a/2)(1,-1,c/a),  v2 = (a/2)(1,1,c/a),  v3 = (a/2)(-1,-1,c/a).


Now, I don't think that these two are consistent with the same convention
for the choice of basis. They are not both right-handed. Also, The BCT
does not have the same relative internal angle between two basis vectors
(their scalar products are different). Third, it doesn't work ;)

Please, try to specify the atomic positions inside the cell using the flag
{crystal}. Then check the results on xcrysden. Probably you will get a
weird configuration.

Thanks,

André Saraiva






>
> On Mar 19, 2009, at 19:04 , Andre Saraiva wrote:
>
>> the basis vectors for  the Body Centered Tetragonal Bravais Lattice
>> are strange. I believe the  correct expression for v1 would be
>>
>> v1 = (a/2)(-1,1,c/a)
>
> the choice of crystal axis is not unique. What the documentation says
> is consistent, AFAIK, with what the code does. All axis triplets used
> by PWscf are right-handed, by the way, while yours is left-handed.
>
> Paolo
> ---
> Paolo Giannozzi, Democritos and University of Udine, Italy
>
>
>
>