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

[anz118466 at cse.iitd.ac.in]

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.

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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>
> --
> 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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