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

Mon Jun 23 17:32:38 CEST 2014

Hi Gabriele,

Thank you for the reply.

I am not sure if I understand correctly on trying a regular periodic
calculation...could you shed more light?

I am attaching the zero-bias transmission for just the nanoribbon for your
reference.

Kind Regards,
Anusha

> 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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