[Pw_forum] "not orthogonal operation" during vc-relax when optimizing graphene

[Peng Tao]

Dear all,

Recently, I just want to putting 'vc-relax' to optimize the graphene structure,
but it repots an error:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
     from checkallsym : error #         2
     not orthogonal operation
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

And I set 'cell_dofree = xy' in the vc-relax run.

I know, it is no need to apply vc-relax to graphene, and only relax can aquire 
my result. But if I want to relax bi-layer and mutilayer graphene and my ultimate
goal is to get their phonon dispersion curves(so the symmetry should not be broken),
how could I do? Is there any convienient method to relax them?

Thanks a lot.

Yours,
Peng Tao

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PH.D. candidate Peng Tao 
Magnetism and Magnetic Materials Division
National Laboratory for Material Science
Institute of Metal Research, Chinese Academy of Sciences
Phone  +86-024-83978751
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