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

anz118466 at cse.iitd.ac.in anz118466 at cse.iitd.ac.in
Tue Jun 10 12:19:35 CEST 2014


Hi Gabriele,

As suggested, I tried to simulate with a smaller system in the 32-bit
system. It produced no error (with and without dispersion correction). I
believe it must be due to the larger system that the allocation error
occurred. Any way to circumvent this?

Thanks & Regards,
Anusha Iyer



> 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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>>> DFTB-Plus-User at dftb-plus.info
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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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> DFTB-Plus-User at dftb-plus.info
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>




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