The calculation did not converge when the scattering region is long

[Yueyang]

Hi,
I optimized a unit cell of graphene nanoribbon, and then repeat it to different lengths to calculate IV curves with the DFTB method.
However, I found that when the scattering region is long, 15 unit cells for example, the calculation is unable to converge to the requested tolerance (e-05) even when the maximum steps is set to be 1000.
But the problem is absent when the scattering region is short, 7 unit cells for example.
Is this normal?
Does that mean we can only study short structures with the software?

Thank you

[Jess Wellendorff]

No, that should not be the case. ATK works for large structures also. However, SCF convergence can sometimes become tricky for very large structures, especially at large bias. Can you attach the script and logging output? Thanks.

[Yueyang]

Here are two of the scripts that can't converge. The output files are too large to upload.

By the way, is it useful to tune the "Damping factor" or the "History steps" to get faster convergence?
Please give me more advices.
Thank you very much!

[Jess Wellendorff]

Perhaps it is the spin-polarization that give you trouble. I suggest non-polarized calculations. The damping factor is already quite low, but you could try to decrease the number of history steps to e.g. 10.

[Yueyang]

OK, I will try it.
Thank you!