[DFTB-Plus-User] Correctness of SCC convergence in DFT+NEGF

[anz118466 at cse.iitd.ac.in]

Hi Gabriele,

For the second testcase, I had kept the poisson box as (30,30) Angstroms
along the x and z directions. The transport direction is along y-axis and
the 'MinimalGrid' was 0.2. These settings gave the weird convergence trend
I had shared before.

I repeated the simulation with a bigger poisson box (50,50) Angstroms
along x and z axis, and a 'MinimalGrid' of 0.3. No convergence was
achieved even after 700 iterations. In both the cases, the broyden
'MixingParameter' was kept as 0.3 and 'Temperature' as 300K. I am at a
loss on how to proceed now.

I am attaching the .gen file for your reference and also, the detailed.out
for the (30,30) poisson box simulation. Kindly have a look. Any feedback
would be very helpful.

Thanks and Regards,
Anusha





> Hi Anusha,
>
> the Poisson box may indeed play a very important role in the convergence
path.
>
> When everything is properly setup, equilibrium calculation are very
often similar to a periodic dftb+ calculation, an SCC cicle of 14 steps
is fine (a bit fast, even).
> Hundreds of steps is definitely long and I would be suspicious as well.
Also the SCC error gets very large.
>
> My experience is that a small Poisson box (along any direction) can be
responsible for it. The mixer is quite robust, therefore I observed that
with a small Poisson box you can still reach convergence to the exact
solution but with very long and weird SCC steps. I suggest you to check
that, in case of non-periodic calculation, you have enough vacuum around
the device (if you can reach it computationally, a couple of nm on each
side for each direction). The minimal grid step default is usually fine,
at worst try to reduce it slightly (0.3).
>
> I rarely observed convergence to the trivial zero solution, i.e. all the
electrons are shot away. I suggest you to check the detailed.out and
verify that the charge population is reasonable.
>
> Hope this helps
> Gabriele
>
>
> On 06/20/2014 03:58 PM, anz118466 at cse.iitd.ac.in wrote:
>> Dear all,
>>
>> I have simulated two testcases on the DFTB+NEGF. Both consist of a
biomolecule on a GNR. I observed two very different trends in SCC
convergence while simulating for the transmission spectra, which I am
sharing here.
>>
>> The first case consists of 1795 atoms in total and saw an extremely
fast convergence in just 14 steps. The second one, however, had 604
atoms and saw convergence in around 600+ steps. Also, the SCC error in
the second case increased gradually first and then decreased rapidly.
For example, the SCC error reached a high of 0.21776430E+03 in 402
steps. It then gradually decreased to 0.31423479E+02 in another 200
steps, and fell sharply to 0.15680539E-05 in another 5 steps. The value
of
>> 'SCCTolerance'
>> was kept 1e-4 in both the cases.
>>
>> Looking at these two trends, I am confused with the correctness of the
final output. How do I ensure that no numerical noise has crept in any
of
>> my two testcases? Also, I observe that the final output changes by
modifying the poisson box dimensions. Is there any rule of thumb
regarding
>> the choice of the poisson box? Please let me know if you need more data
from me for further clarification.
>>
>> Thanks & Regards,
>> Anusha Iyer
>>
>>
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>
>
> --
> Dr. Gabriele Penazzi
> BCCMS - University of Bremen
>
> http://www.bccms.uni-bremen.de/
> http://sites.google.com/site/gabrielepenazzi/
>
> phone: +49 (0) 421 218 62337
> fax: +49 (0) 421 218 62770
>
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