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

Tue Jun 24 09:11:55 CEST 2014

Hi Anusha,

no, I mean a calculation without transport, Poisson solver and Green's
function.
Just a plain supercell DFTB calculation in the Gamma point solved by
diagonalization.

Best,
Gabriele

On 06/23/2014 08:44 PM, anz118466 at cse.iitd.ac.in wrote:
> 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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>>>>>>> DFTB-Plus-User at dftb-plus.info
>>>>>>> http://www.dftb-plus.info/mailman/listinfo/dftb-plus-user
>>>>>>>
>>>>>> --
>>>>>> 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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>>>> --
>>>> 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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>>
>> --
>> 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