[QE-users] "Eigenvectors exchange needed " in the ph.x calculation

[Gang Yang]

Dear users and experts: 

I carried on a phonon calculation for a 20 atom strucuture using QE ph.x. However, in the ph.x output file, it show some messages as follow: 
"Dynamical matrices for ( 2, 2, 2) uniform grid of q-points 
( 6 q-points): 
N xq(1) xq(2) xq(3) 
1 0.000000000 0.000000000 0.000000000 
2 0.000000000 0.000000000 -0.397359503 
3 -1.222792984 -0.255394699 0.000000000 
4 -1.222792984 -0.255394699 -0.397359503 
5 -2.445585969 0.000000000 0.000000000 
6 -2.445585969 0.000000000 -0.397359503 
Eigenvectors exchange needed 1 31 
Eigenvectors exchange needed 3 33 
Eigenvectors exchange needed 5 35 
Eigenvectors exchange needed 7 37 
Eigenvectors exchange needed 9 39 
Eigenvectors exchange needed 11 41 
Eigenvectors exchange needed 13 43 
Eigenvectors exchange needed 15 45 
Eigenvectors exchange needed 17 47 
" 
I have two questions: 
1.) I don't know that what exactly " Eigenvectors exchange needed " mean? 
2) Will it cause a negative frequencies？ 
Thank you. 

Regards, 
Yang Gang 
Institut des Nanotechnologies de Lyon (INL) - UMR5270 
Ecole Centrale de Lyon 
36, Avenue Guy de Collongue 
69 134 Ecully Cedex 
Tél.: +33 (0)7 64 83 01 70 

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