[DFTB-Plus-User] Dispersion correction in DFTB+NEGF

[Gabriele Penazzi]

On 06/10/2014 07:52 AM, anz118466 at cse.iitd.ac.in wrote:
> Hi Gabriele,
>
> I tried to simulate a testcase I have been running on another machine
> (64-bit). The 64-bit machine gives no such error, but the current machine
> exits with the error.
>
> I believe that the simulation works in 64-bit, but the current machine is
> a 32-bit one (i686). I had compiled the i686 DFTB+ from the source few
> days back. When I run the transport simulations, the dftb+ gives no error
> for the source/drain calculations. However, when I try to run the device
> input file, it exits with the 'ALLOCATION ERROR'. This 32-bit machine has
> 8 cores and 24GB RAM.
>
> In addition to above, I had a more general query. In the course of
> performing DFTB+ calculations, I have realised that the SCC convergence
> highly depends on the input geometry. For small geometries, it would make
> sense to optimise in DFTB+ first. But, I am handling a 1500+ atom system.
> Kindly share how an optimised geometry can be generated in such a case to
> achieve convergence.

Dear Anusha,

I don't see how this is related to the dispersion correction. You should
try a SMALL test case, to see if you can reproduce the bug with the
minimum amount of information possible. Moreover, I can not estimate if
your memory is enough or not without details on your system, an
allocation error may be due as well to insufficient memory. The
source/drain calculation is typically much less memory demanding.

Unluckily there is no brute force way to relax a large structure,
however you could be able to relax a 1500 atoms structure with the
parallel version of DFTB+. Otherwise you'll have to rely on cheaper
methods for the relaxation, like a classical force field.

Best
Gabriele


>
> Thanks and Regards,
> Anusha
>
>
>
>> On 06/08/2014 03:25 PM, anz118466 at cse.iitd.ac.in wrote:
>>> Dear all,
>>>
>>> I had a small query regarding the DFTB+NEGF package. Is dispersion
>>> correction supported for the same? The DFTB+NEGF manual mentions about
>>> the
>>> use of SlaterKirkwood model. Can I include this in my transport
>>> calculations for considering van der waals forces?
>> Dear Anusha,
>>
>> we are generally trying to keep all the usual DFTB+ features in the
>> transport trunk, therefore we did not specifically remove anything for
>> dispersion correction but neither we tested a case like you mention.
>>
>> The reason is that van der Waals forces are long electrostatic forces
>> and they do not enter directly the electronic Hamiltonian, therefore
>> they do not affect charge transport. One case I could imagine would be
>> if you want to calculate forces out of equilibrium and include also
>> dispersion correction, but such a feature is not supported yet.
>>
>> Going back to the allocation error, which should not be there in any
>> case, can you give us some more information?
>> How large is the system? Did you try a small test case? Does the normal
>> transport calculation without SlaterKirkwood correction work properly?
>>
>> Best,
>> Gabriele
>>
>>
>>> Also, when I incorporated the dispersion block in my dftb+negf
>>> simulation,
>>> I encountered the following error:
>>>
>>> ALLOCATION ERROR
>>>
>>> Is this due to some memory shortage?
>>>
>>> Thanks & Regards,
>>> Anusha Iyer
>>>
>>> _______________________________________________
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>>> DFTB-Plus-User at dftb-plus.info
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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