[Wien] Question on EFG and eigenvectors and sugestion for the output

[Florent Boucher]

Thank you Stephan for your kind help.
Stefaan Cottenier a écrit :
>> Now what about the eigenvectors or principle axis of the tensor.
>> Is the direction of the  principle axis defined without ambiguity ?
>> For instance, if I have an atom on a site with a local symmetry -1. I
>> was assuming that the principle axis of the EFG tensor can be either in
>> one direction or the opposite ?
>> Am I true ?
>> If not, why ?
>>     
>
> You can change the direction of the axis, yes. The problem of finding  
> the principle axis system for an EFG is mathematically equivalent to  
> finding the principle axes of a 3D ellipsoid. Obviously you can have  
> the freedom there as well to invert your axes.
>
> You can convince yourself about this by plotting a 1/r potential in  
> one dimension, as well as the corresponding field (=the first  
> derivative) and the gradient of the field (the second derivative of  
> the potential). The latter will turn out to be symmetrical with  
> respect to an inversion of the direction of your axis.
>
>   

I was arrived at the same conclusion as your (looking the EFG tensor 
like an ellipsoïd), I just wanted to be sure.
It means that in such case there are at least four equivalent choices of 
eulerian angles to orient the EFG tensor in the cartesiane set of axis.
The same is true for the NMR CSA tensor (that can be calculated with the 
GIPAW method).
So, it can be conclude that they are 16 equivalent choices of Eulerian 
angles if you want to orient the EFG tensor with respect to the CSA one.
I am not sure that people that use NMR and want to make comparison doing 
DFT calculations are aware of this specific point.


Stefaan Cottenier a écrit :
> Do you really think this is useful? It's a multiplication by  
> fundamental constants only (e/h), and the quantity that really  
> determines the value is Q, which you have to specify for any  
> individual case anyway...?
>   
On the last point for the output of lapw0, it is just a way to help 
users. If you don't do very often EFG calculations, you will have to 
remember every time how to convert the Vzz into a Cq quantity.
I know that PWSCF, Paratec and Castep do the Cq calculation for you if 
you give Q in barn. I was just finding this option very convenient for 
the user (and not too complicated to implement...)
Regards
Florent

-- 
 -------------------------------------------------------------------------
| Florent BOUCHER                    |                                    |
| Institut des Matériaux Jean Rouxel | Mailto:Florent.Boucher at cnrs-imn.fr |
| 2, rue de la Houssinière           | Phone: (33) 2 40 37 39 24          |
| BP 32229                           | Fax:   (33) 2 40 37 39 95          |
| 44322 NANTES CEDEX 3 (FRANCE)      | http://www.cnrs-imn.fr             |
 -------------------------------------------------------------------------

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://zeus.theochem.tuwien.ac.at/pipermail/wien/attachments/20071207/ad61ec69/attachment.html