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

Gabriele Penazzi gabriele.penazzi at bccms.uni-bremen.de
Sun May 11 10:50:14 CEST 2014 

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 
> <mailto: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 <mailto:Engineering at K.U>. Leuven
>>     BIOTECH at TU Dresden
>>     Email:liuenlong20 at gmail.com <mailto:liuenlong20 at gmail.com>;
>>     enlong.liu at student.kuleuven.be
>>     <mailto:enlong.liu at student.kuleuven.be>;
>>     enlong.liu at biotech.tu-dresden.de
>>     <mailto:enlong.liu at biotech.tu-dresden.de>
>>     Mobile Phone: +4917666191322 <tel:%2B4917666191322>
>>     Mailing Address: Zi. 0108R, Budapester Straße 24, 01069, Dresden,
>>     Germany
>>
>>
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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  <tel:%2B49%20%280%29%20421%20218%2062337>
>     fax:+49 (0) 421 218 62770  <tel:%2B49%20%280%29%20421%20218%2062770>
>
>
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>
>
>
> -- 
> Faculty of Engineering at K.U. Leuven
> BIOTECH at TU Dresden
> Email:liuenlong20 at gmail.com <mailto:liuenlong20 at gmail.com>; 
> enlong.liu at student.kuleuven.be 
> <mailto:enlong.liu at student.kuleuven.be>; 
> enlong.liu at biotech.tu-dresden.de <mailto:enlong.liu at biotech.tu-dresden.de>
> Mobile Phone: +4917666191322
> Mailing Address: Zi. 0108R, Budapester Straße 24, 01069, Dresden, Germany
>
>
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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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