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Thank you Stephan for your kind help.<br>
Stefaan Cottenier a écrit :
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<pre wrap="">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 ?
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<pre wrap=""><!---->
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.
</pre>
</blockquote>
<br>
I was arrived at the same conclusion as your (looking the EFG tensor
like an ellipsoïd), I just wanted to be sure.<br>
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.<br>
The same is true for the NMR CSA tensor (that can be calculated with
the GIPAW method).<br>
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.<br>
I am not sure that people that use NMR and want to make comparison
doing DFT calculations are aware of this specific point.<br>
<br>
<br>
Stefaan Cottenier a écrit :
<blockquote type="cite">
<pre wrap="">
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...?
</pre>
</blockquote>
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.<br>
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...)<br>
Regards<br>
Florent<br>
<br>
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