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

[Gabriele Penazzi]

On 06/23/2014 05:32 PM, anz118466 at cse.iitd.ac.in wrote:
> 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?

Hi Anusha,

I mean a DFTB calculation with periodic boundary conditions along
transport direction instead of a transport calculation with open
boundary conditions.
If it doesn't converge with a periodic calculation, hardly it will
converge with open boundaries. I'm saying to try it because physisorbed
or weakly bounded systems are dangerous beasts in DFTB

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

It's a semiconducting system, that's why increasing temperature to 300K
doesn't help.
>
> 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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> >
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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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