[Pw_forum] Inconsistencies in the CELL_PARAMETERS - ibrav=5
Stefano de Gironcoli
degironc at sissa.it
Thu Dec 23 19:49:13 CET 2010
if you are interested in comparing with the definition of in INPUT.PW
you should apply the definition in INPUT.PW...
which is not the one you are using.
There is a value of "a" missing in your definition.
stefano
Marino Vetuschi Zuccolini wrote:
> 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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