Dear Chucui,

Since you haven’t provided a minimal working example, I could only speculate 
what might be the issue. From the incomplete example that you’ve shown here, I 
would guess that this might be the problem:

> On 05 Aug 2018, at 09:28, [email protected] wrote:
> 
>                       GridTools::collect_periodic_faces(dof_handler,
>                                   /*b_id1*/ 0,
>                                   /*b_id2*/ 1,
>                                   /*direction*/ 0,
>                                   periodicity_vector);
>           GridTools::collect_periodic_faces(dof_handler,
>                                   /*b_id1*/ 2,
>                                   /*b_id2*/ 3,
>                                   /*direction*/ 0,   <——— Should be a "1"
>                                   periodicity_vector);

For the unit cube coarse face pairs {0,1} should have their normal in the 
x-direction, and coarse face pairs {2,3} would then have their normal vectors 
orientated in the y-direction.

If not then it *might* be that your coarsest grid needs to have more than one 
cell. However, looking at the documentation to step-45 
<https://dealii.org/9.0.0/doxygen/deal.II/step_45.html> (specifically the 
introduction), I don’t believe this to be the case. You can try to take the 
coarsest grid, refine it globally once, and then use the 
GridTools::flatten_triangulation() 
<https://dealii.org/9.0.0/doxygen/deal.II/namespaceGridGenerator.html#afe86bf1a4502131ec811a6de41ab41e9>
 function to make this the new coarsest grid.

I hope that this helps you solve the problem.

Best,
Jean-Paul



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