[Pw_forum] A bug in the pp.x?

[Tone Kokalj]

On Sun, 2008-06-08 at 23:29 +0800, zhaohscas wrote:
> >Hi chopstick ;-),
> 
> Aha, I'm using both forks and chopsticks once in a while.
> 
> >>  In the abobe case, the e1 \bot e2, and e2 \bot e3, but e1 isn'
> >>  perpendicular to e3.  So, this will violate orthogonal condition.
> >
> >You are right: the actual check performed by the code is:
> >  e_i \cdot \e_j > 1.d-6
> >which wrongly allows non-orthogonal vectors if their pairwise scalar
> >products are negative. Thank you for reporting, it will be fixed in the
> >next release.
> 
> But, in my mind, as for the study of the distribution of the charge density, we generallly want to 
> have the distribution w.r.t a specific miller plane.  In the periodic crystal system, this can be 
> represented on the plane determined by a parallelogram whose two sides are the corresponding 
> primitive plane vectors.  If we consider this requirement, I think we should use miller plane 
> vectors to contruct the grid, which are not always perpendicular indeed.  Under the above consideration,  
> the orthogonal condition will be not so good.

I believe the primary reason for orthogonality constraint was/is that
not all drawing software can handle non-orthogonal mesh. Otherwise your
considerations are justified: in many instances non-orthogonal 2D region
would make sense. 

I belive orthogonality constraint could be lifted at least for XSF
formatted output file.

Regards, Tone
-- 
Tone Kokalj <tone.kokalj at ijs.si>
J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia (tel:
+386-1-477-3523 // fax:+386-1-477-3822)