[Pw_forum] ibrav=3 crystal axes

[Paolo Giannozzi]

On Wed, Jan 25, 2017 at 8:48 AM, Mihalis Kavousanakis
<mihalis.kavousanakis at scienomics.com> wrote:
> Dear all,
>
> I have a question regarding ibrav=3 crystal axes, which are reported as
> v1=(a/2)(1,1,1), v2=(a/2)(-1,1,1), v3=(a2/2)(-1,-1,1)
>
> Should these be rather:
> v1=(a/2)(-1,1,1), v2=(a/2)(1-,1,1), v3=(a2/2)(1,1,-1) ?

they should, but they aren't, for reasons nobody knows any longer
since many years (that part of the code likely dates back to the
70's). Unfortunately it is not possible to change it without breaking
existing input data. The only thing one might do is to add a
"ibrav=-3" case with a more sensible choice of axis.

> I encountered this problem when I performed an scf simulation on a
> Molybdenum Im-3m crystal in its primitive form.
> The crystal's dimensions are a=b=c=2.725Angstrom, and
> alpha=beta=gamma=109.47 deg.
>
> The crystal axes as reported in the output file however would give:
> a=b=c=2.725Angstrom, and alpha=beta=70.53deg, beta=109.47 deg.

are you explicitly using the space group and Wyckoff positions? and
are you getting wrong positions? or just a funny set of alpha beta
gamma angles?

Paolo

> I am runnning on QE 5.2.1
>
>
> kind regards,
> Mihalis Kavousanakis
>
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222