[DFTB-Plus-User] Query on the use of DFTB+ for periodic transport calculations

[Bálint Aradi]

Dear Karen,

welcome on board! :-)

I think, the main reason for the observed Fermi level (and band 
structure) shift lies in the charge fluctuations. When you calculate the 
periodic case, the charge transfer between the atoms is nearly zero (as 
it should in pristine gold). When you calculate the non-periodic case 
(or alternatively calculate the periodic case with some vacuum in each 
direction), you introduce charge oscillations due to the free standing 
edges. This results in a Coulomb potential, which is negative on each 
atom and shifts the band structure accordingly downwards.

Best regards,

Bálint


Ps. When making the contact calculations, make sure to specify a decent 
k-point sampling also along the transport direction, as DFTB+ will make 
your periodic along that direction as well (using one PL as unit cell).


On 02.03.23 15:13, Karen Schafer wrote:
> Dear mailing list members,
> 
> I am relatively new to using DFTB+ and I can't fix a problem with 
> self-consistent, periodic transport calculations. I am investigating a 
> system with two gold (Au(111)) surfaces, bridged by an alkanedithiol 
> ligand. The system is supposed to be periodic in x and y direction and I 
> added a vacuum gap in z direction (transport direction).
> 
> When calculating the contacts, I receive a fermi energy of -2.6 eV which 
> is higher than I would expect and also higher than what I get from 
> non-periodic conditions (-4.2 eV), which results in a shift of the 
> transmission compared to non-periodic and non SCC calculations.
> 
> I have included the gen structure file and the dftb_in file for the 
> contact calculation. I have tried to fix the fermi energy by increasing 
> the kpoints but that did not help. Is there something I am doing wrong?
> 
> Thank you in advance for your time and effort,
> 
> Karen
> 
> 
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-- 
Dr. Bálint Aradi
Bremen Center for Computational Materials Science, University of Bremen
http://www.bccms.uni-bremen.de/cms/people/b-aradi/


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