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

[Enlong Liu]

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?

3. I am not very clear about this tip, since I don't know where can I get
the reference.

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
>
>
> _______________________________________________
> DFTB-Plus-User mailing list
> DFTB-Plus-User at dftb-plus.info
> http://www.dftb-plus.info/mailman/listinfo/dftb-plus-user
>
>


-- 
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.zfn.uni-bremen.de/pipermail/dftb-plus-user/attachments/20140511/36573300/attachment.htm>