[Pw_forum] wrong phonon eigenvectors.

[Vasile Chis]

Dear all,
I'm performing phonon calculations of a 7-layer Cu(111) slab using a PBE 
ultrasoft pseudo potential.
ecutwfc=30 Ry, ecutrho=480 Ry, smearing=mp, conv-thr=1.0d-12 over a 
12x12x1 MP-mesh, ibrav=4.

The slab has been relaxed untill total force in the slab was 
0.77meV/Angstrom.
The phonon calculation converges with no problem.
I'm using phonon thresholt of 1.0d-14 and a alpha-mix of 0.1

The main problem is not at Gamma, but at the K-point of the SBZ.
K = (0.0,  1/sqrt(3),  0.0) in a_lat units.
At this point the frequencies ar correct comparing with experimental 
results.
What bothers me is that the eigenvectors of the lowest mode are incorrect.
A sample of the lowest mode:

     q = (    0.000000000   0.577350300   0.000000000 )

 ****************************************************
     omega( 1) =       3.195483 [THz] =     106.590533 [cm-1]
 (  0.000000  0.000000  0.067560  0.000000  0.458240  0.000000 )
 (  0.000000  0.000000 -0.139989  0.000000 -0.322556  0.000000 )
 (  0.000000  0.000000 -0.115517  0.000000 -0.308861  0.000000 )
 (  0.000000  0.000000 -0.108002  0.000000 -0.307379  0.000000 )
 (  0.000000  0.000000  0.115517  0.000000  0.308861  0.000000 )
 (  0.000000  0.000000  0.139989  0.000000  0.322556  0.000000 )
 (  0.000000  0.000000  0.067560  0.000000  0.458240  0.000000 )

I have not check if the rotational ivariance holdes.
If it does not hold, that means that the Rayleigh wave have been lost.
Still if the RW is lost how does one find it back?

What could be the problem of wrong eigenvectors and how does one solve it?
Is the problem of to few layers in the slab?

One other thing, I would expect, during the phonon calculation of a high 
symmetry point on the boundary edge, to only have 14 irreducible 
representations. But in my case a have all 21.

I would be delighted if some one have some pointers to this problem.
Thank you in advance.

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Vasile Chis
Interface Modelling Group
Solid State Physics
Department of Experimental Physics
Göteborgs Universitet/Chalmers

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