[Pw_forum] monoclinic cell definition question

[jasius_1 at yahoo.com]

thanks. the problem is that the new gamma between new a and b is still 90o!

Right now in the cif file I have a unique angle between a and c, while angles between a and b and b and c are 90o. According to the pwscf manual, the only angle I need to specify is between a and b (that's why I started this rotation question). but if I rotate everything once then this angle becomes between rotated vectors b and c, still not a and b as requested in the manual.

shall I rotate twice, e.g. 240 degrees? then the new vectors comprising the unique angle would actually be a and b, as requested in the manual. victory shall be mine!

the problem with transofrming xyz is that I work with cartesians. I can get them easiliy and, to admit the truth, I still can't figure out the difference between alat and crystal coords. just can;t. read the Wyckoff file on the manual page, can't understand the difference

is alat simple fractional coords? can they be simply rotated twice then?

finally, what is a good check that all the rotations were valid?

Jonas

--- On Wed, 5/13/09, Lorenzo Paulatto <paulatto at sissa.it> wrote:

> From: Lorenzo Paulatto <paulatto at sissa.it>
> Subject: Re: [Pw_forum] monoclinic cell definition question
> To: jasius_1 at yahoo.com, "PWSCF Forum" <pw_forum at pwscf.org>
> Date: Wednesday, May 13, 2009, 2:36 AM
> 
> On Wed, May 13, 2009 03:08, Jonas Baltrusaitis wrote:
> >
> >  wait, but then it's not clear anymore what
> angles are between which
> >  axes...
> >
> >  Jonas
> 
> 
> The "new" gamma is between the new a and the new b, while
> the "old" gamma
> is between the old a and the old b. It is just a
> redefinition, you are not
> changing your system your are just changing point of view.
> 
> On the x,y,z swapping: it is a bit complicated to redefine
> the
> coordinates, at least I'm not sure it is a simple
> permutation. You've
> better define the atoms positions in "crystal" units, in
> this way you can
> just apply the same permutation you used to redefine the
> axis.
> 
> But there may be a simpler solution: use ibrav=0 and define
> your axis by
> hand according to your preferred convention. It may sound
> dirty, but it is
> not: the code can still find all the symmetries
> operations.
> 
> regards
> 
> -- 
> Lorenzo Paulatto
> SISSA  &  DEMOCRITOS (Trieste)
> phone: +39 040 3787 511
> skype: paulatz
> www:   http://people.sissa.it/~paulatto/
> 
> 
> 
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