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

[Gabriele Penazzi]

Hi Anusha,

not an easy system, I'd say. Could you try a 'regular' DFTB periodic
calculation? Weakly bounded systems can be tricky even without
transport, you should check whether you get a reasonable energetics with
a periodic calculation.

The broyden is a bit fast, at most I would lower the value (0.05), but I
don't think it will help. If you have a periodic result, you may try to
set an initial charge value for every atom in the transport calculation.

I think that the Poisson box is large enough. Is the nanoribbon metallic?

Best,
Gabriele



On 06/23/2014 04:42 PM, anz118466 at cse.iitd.ac.in wrote:
>
> 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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>
>
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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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