[QE-users] band convergece problem in charged system

[Leo Yang]

Dear Thomas,
  
 Thanks for your reply. I tried the neutral system before the charge one, which is simulated with Martyna-Tuckerman correction. The good thing is the band gap could achieve convergence in respect of super cell size. And I found that there is band gap between band 104 and 105 about 3.3 ev. But the occupation number of band 92-104 is fractional. The reason for that is there are lack of 4 electrons to occupy the band meanwhile I use smearing for the calculation. After adding 4 additional electrons to the system as I referred in the last email with the convergence size of supercell tested under neutral system (20 Angs), the band 92-104 are full occupation but decrease in band gap.
  
 So I thing the reason leads my problem may be relative to "curvature" in vacuum space, which you mentioned before. If it is, do you have any solution for that? Thank you.
  
 Best wishes
 Yang
  
  
 --------
  
Yang Zhou
 
PhD student
 
University of Leeds
 
United Kingdom


  

 

 ------------------ Original ------------------
  From: "Thomas Brumme"<thomas.brumme at uni-leipzig.de>;
 Date: Tue, Oct 22, 2019 10:37 PM
 To: "Quantum ESPRESSO users Forum"<users at lists.quantum-espresso.org>;"Leo Yang"<121076797 at qq.com>;
 
 Subject: Re: [QE-users] band convergece problem in charged system

 

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

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