[DFTB-Plus-User] Different Fermi energy calculated by DFTB+NEGF and only DFTB+

Sun May 11 11:47:07 CEST 2014

Dear Gabriele,

In the attachment is the file I used for calculation.

Thanks a lot for your help!

Best regards,

2014-05-11 16:50 GMT+08:00 Gabriele Penazzi <
gabriele.penazzi at bccms.uni-bremen.de>:

>  On 05/11/2014 10:31 AM, Enlong Liu wrote:
>
> Dear Gabriele,
>
>  I checked my system according to your suggestion in these days, Here is
> the feedback:
>
>  1. My system is metal. Since my system is a nanotube standing vertically
> on a part of Ag(111) slab and I want to get the transmission function
> through the contact, one of the contact is part of the Ag slab.
>
>  2. You mean that in contact calculation of Ag slab, the k point and
> weights should be the same as DOS and band structure calculation? But in
> the latter, the total energy and Fermi energy changes according to the k
> points. I think maybe convergence should be reached in this case to
> determine the k point?
>
>
> Yes, if you compare two calculations the k-point sampling should be the
> same. Sometimes this can be confusing in 1D systems due to the way the
> k-sampling is defined in the transport input file, but there should be no
> confusion for a periodic case.
>
> I am not sure what you mean for band structure calculation, as in this
> case you first calculate the self-consistent charge density with a given
> k-point sampling (the one which matters for getting the right solution) and
> then the band structure with a non-scc calculation and a different k-point
> sampling (the one which matters only for fixing the coordinates of your
> plot in reciprocal space).
>
> Also I didn't understand if your Fermi level is consistent respect to the
> DOS. Could you send the input files you use for the two calculations?
> For periodic structure, there could be some issue due to the reference
> potential in the poisson solver, which could be solved by using the poisson
> solver for the contact calculation as well. Anyway, if you send the input
> files I can try to verify if the issue is in the input or in the code.
>
>
>
>
>  3. I am not very clear about this tip, since I don't know where can I
> get the reference.
>
>
> I just mean that if you compare the contact calculation and a periodic
> dftb calculation, do it using the same version of the code. Fermi level can
> differ, the important is that they are consistent with the DOS and with the
> rest of the calculation.
>
>
>  Thanks!
>
>  Kind regards,
>
>
> 2014-05-09 22:53 GMT+08:00 Gabriele Penazzi <
> gabriele.penazzi at bccms.uni-bremen.de>:
>
>>  On 05/08/2014 05:06 PM, Enlong Liu wrote:
>>
>> Dear all,
>>
>>  I am doing transport calculation to boron nanotubes (BNT). But I find
>> out that there is a huge difference in fermi energy in different cases.
>>
>>  In the transport calculation, I used DFTB+NEGF to calculate the contact
>> firstly, which is part of BNT at one end and the result is -10eV
>> approximately. But if I just calculate the DOS or band structure to the
>> same BNT part, the fermi energy is just -4eV. It is interesting since I
>> think all other parameters are the same, only the calculation situation is
>> different, one is just DFTB+ for DOS, the other includes NEGF to calculate
>> contact properties before doing transport calculation.
>>
>>  I tried some other system like Si or Ag. There is the same problem,
>> fermi energy difference in common calculation and transport calculation.
>> Could any one help me with that?
>>
>>
>>  Dear Enlong,
>>
>> we need to be a bit careful here as there is a couple of issues.
>>
>> First: is the system a semiconductor or a metal or a degenerate
>> semiconductor?
>> In the first case, as long as the Fermi level is in the gap, at zero
>> temperature there's no difference between quantitative differences in the
>> numerical value.
>>
>> Second: be careful with the k point sampling, sometimes this can look
>> confusing. Check the manual to verify that your k-points are consistent
>> between dos and band structure calculation.
>>
>> Third: compare consistently. I am not sure whether it makes difference or
>> not, but to be safe just compare quantities you get with the same version
>> of the code as the Fermi level, being an energy, is defined up to a given
>> reference.
>>
>> Fourth: if all the above is verified, in some specific cases there could
>> be still unknown issues. My experience is that in non-periodic systems
>> everything works well. In periodic systems there could be more delicate
>> cases due to the different boundary conditions in the poisson. Nevertheless
>> the huge difference (6 eV) makes me believe that you are in one of the
>> previous cases. It is a good idea that you check your equilibrium
>> calculation and verify that at the interface between contact and device
>> region discontinuities in the potential are not large, because that is
>> often a signature of a wrong fermi level.
>>
>> Regards,
>> Gabriele
>>
>>
>>
>>  Thanks a lot!
>>
>>  Best regards,
>> Enlong
>>
>>  --
>>  Faculty of Engineering at K.U. Leuven
>> BIOTECH at TU Dresden
>> Email: liuenlong20 at gmail.com; enlong.liu at student.kuleuven.be;
>> enlong.liu at biotech.tu-dresden.de
>> Mobile Phone: +4917666191322
>> Mailing Address: Zi. 0108R, Budapester Straße 24, 01069, Dresden, Germany
>>
>>
>>  _______________________________________________
>> DFTB-Plus-User mailing listDFTB-Plus-User at dftb-plus.infohttp://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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>>
>>
>
>
>  --
>  Faculty of Engineering at K.U. Leuven
> BIOTECH at TU Dresden
> Email: liuenlong20 at gmail.com; enlong.liu at student.kuleuven.be;
> enlong.liu at biotech.tu-dresden.de
> Mobile Phone: +4917666191322
> Mailing Address: Zi. 0108R, Budapester Straße 24, 01069, Dresden, Germany
>
>
> _______________________________________________
> DFTB-Plus-User mailing listDFTB-Plus-User at dftb-plus.infohttp://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
>
>
> _______________________________________________
> DFTB-Plus-User mailing list
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>
>

-- 
Faculty of Engineering at K.U. Leuven
BIOTECH at TU Dresden
Email: liuenlong20 at gmail.com; enlong.liu at student.kuleuven.be;
enlong.liu at biotech.tu-dresden.de
Mobile Phone: +4917666191322
Mailing Address: Zi. 0108R, Budapester Straße 24, 01069, Dresden, Germany
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