# [Pw_forum] Inconsistencies in the CELL_PARAMETERS - ibrav=5

Marino Vetuschi Zuccolini zucco at dipteris.unige.it
Thu Dec 23 19:43:13 CET 2010

```stefano,
thanks but can you explain a little bit more exhaustively, because I do not understand. It seems that tx and ty and the cos(alpha) derived from them are numerically exact and I suppose the algorithm are right.

...
v_1=sqrt(at(1,1)**2+at(2,1)**2+at(3,1)**2)
cell_edge=alat*v_1
c_tx=(1-2*(at(1,1))**2)
c_ty=(-6*at(2,1)**2)+1
c_tz=((3*at(3,1)**2-1)/(2))
....

tx=sqrt((1-c)/2)
ty=sqrt((1-c)/6)
tz=sqrt((1+2*c)/3)
...

m.

On 23 Dec 2010, at 17:28, Stefano de Gironcoli wrote:

> Dear  Marino Vetuschi Zuccolini wrote:
>> trigonal(r)
>>                   ===================
>>                   for these groups, the z-axis is chosen as the 3-fold axis, but the
>>                   crystallographic vectors form a three-fold star around the z-axis,
>>                   and the primitive cell is a simple rhombohedron. The crystallographic
>>                   vectors are:
>>                         v1 = a(tx,-ty,tz),   v2 = a(0,2ty,tz),   v3 = a(-tx,-ty,tz).
>>                   where c=cos(alpha) is the cosine of the angle alpha between any pair
>>                   of crystallographic vectors, tc, ty, tz are defined as
>>                        tx=sqrt((1-c)/2), ty=sqrt((1-c)/6), tz=sqrt((1+2c)/3)
>>
>
> in order to extract c you are forgetting to divide v1 by a=|v1|.
>
> stefano
>
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*******************************************************
Marino Vetuschi Zuccolini
zucco at dipteris.unige.it
Researcher / Geochemist
Laboratory of Geochemistry

DIPartimento per lo studio della TErra e delle sue RISorse - Università  di Genova
Tel. ++39 010 3538136 Fax. ++39 010 352169
Corso Europa 26, 16132 - Genova - Italy

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