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

Mon Jun 23 20:44:47 CEST 2014

Hi Garbiele,

I am trying to set the PBC along transport direction by using:

 OverrideBulkBC = Periodic {
        Boundaries = y
    }

I wanted to confirm if this is the right way of doing this.

Kind Regards,
Anusha

> 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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>
>
> --
> 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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