[Pw_forum] monoclinic cell definition question

[Lorenzo Paulatto]

In data 13 maggio 2009 alle ore 15:08:30, <jasius_1 at yahoo.com> ha scritto:
> 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.

I don't know enough about monoclinic systems to answer this question,  
furthermore I don't understand around what you are rotating!

> 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.

Alat are x,y,z coordinates, using alat (celldm(0), or the length of a) as  
the unit of measure; on the other hand crystal expresses the position of  
the ions as a linear combination of the basis vectors.

e.g. from the navy site  
<http://cst-www.nrl.navy.mil/lattice/struk/g0_6.html> where it says
  B1	 = 	+ x1 A1 + 1/4 A2 + z1 A3
it's using crystal coordinates, conversely
  B1	=	[x1 a + z1 c cos(beta)] X + 1/4 b Y + z1 c sin(beta) Z
are cartesian coordinates, they become alat coordinates if you substitute  
a=>1 and b=>b/a, c=>c/a.

regards

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Lorenzo Paulatto
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