[QE-users] band convergece problem in charged system

[Thomas Brumme]

Dear Yang,

I have no clue how the Martyna-Tuckerman correction works but usually one
has problems with systems having such a large total charge since the total
cell needs to be neutral. Some terms are in fact calculated for a charge 
neutral
system resulting in a total potential which has a curvature as if there 
would be
a homogeneous background charge. This constant curvature leads to a
potential energy minimum in the vacuum space between the repeated cells.
If you increase the cell this minimum goes down in energy and, at some 
point,
crosses the Fermi energy and gets occupied.

Long story short:
I have no clue but are you sure that Martyna-Tuckerman correction works
correctly for charged systems?
Is the LUMO really a state of the molecule and not from a state in the 
vacuum?

You could check if everything works as intended for a neutral system...

Regards

Thomas


On 10/2/19 1:46 PM, Leo Yang wrote:
>
> Dear QE users:
>
> I try to use QE-6.4.1 to simulate as single molecule. The system 
> is built in large super cell, and /total_charge=-4/  is added to input 
> file leading to charged system. When I do the convergence test in term 
> of the super cell size (20 Angs - 30 Angs ), the variation for total 
> energy is small: -1038.6784 Ry for 20 Angs and 1038.7388 Ry for 30 
> Angs. But the band energy and Fermi energy (5.23 ev to 3.08 ev) shift 
> visibly. The problem is that the band gap between band 104 (occupied) 
> and 105 (unoccupied) become smaller with the increase of size (about 
> 0.81 ev change to 0.11 ev). It seems the band gap is gradually 
> disappear, which confuses me so much. The input file is attached with 
> this email. It will be very appreciated if you can solve this for me. 
> Thank you.
>
>
> Best wishes
>
> Yang Zhou
>
>
> ------
>
> Yang Zhou
>
> PhD student
>
> University of Leeds
>
> United Kingdom
>
>
> _______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu/quantum-espresso)
> users mailing list users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users

-- 
Dr. rer. nat. Thomas Brumme
Wilhelm-Ostwald-Institute for Physical and Theoretical Chemistry
Leipzig University
Phillipp-Rosenthal-Strasse 31
04103 Leipzig

Tel:  +49 (0)341 97 36456

email: thomas.brumme at uni-leipzig.de

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20191022/11ad0cf7/attachment.html>