Messages

1

General Questions and Answers / Re: DFT+U in two probe system

« on: December 23, 2014, 08:03 »

Why not? That is not true, there are many calculations using MGGA on metals. Ok, most are using TPSS and not TB09, but as far as I have seen from the metals I tried, MGGA works just fine.

Thank you for correcting my view.
I have tried to use MGGA in my two probe system, but it is hard to converge. ???The py file is attached

2

General Questions and Answers / Re: DFT+U in two probe system

« on: December 23, 2014, 07:42 »

The use of DFT+U may be meaningless to treat BDT molecule, because it is a very weak correlated system.

Recently，I have read a paper about Hubbard U.
http://www.sciencedirect.com/science/article/pii/S0022369713003296#
In this paper, the authors set U parameter for C/H/S elements. So does that mean Hubbard U is also feasible in BDT molecule?
Thank you for your kind help!

3

General Questions and Answers / Re: DFT+U in two probe system

« on: December 19, 2014, 15:04 »

I don't know if Meta-GGA has been tried for molecules, but that's another possibility to at least experiment with. It is available in ATK already, and doesn't add to the computation time substantially.

    Thank you for your kind reply! But I am still confused about what you have said. As I know, Meta-GGA isn't suitable for metals (the electrodes are gold), can we use Meta-GGA for BDT molecule and GGA for gold  separately in ATK?

4

General Questions and Answers / Re: DFT+U in two probe system

« on: December 9, 2014, 08:48 »

The use of DFT+U may be meaningless to treat BDT molecule, because it is a very weak correlated system.

    Thank you for your reply!
    By the way, is there any method to improve the HOMO-LUMO gap in ATK? Since the position of energy level is very important in studying electron transfer.

5

General Questions and Answers / DFT+U in two probe system

« on: December 8, 2014, 09:13 »

Hi ATK-VNL team:
        When I calculate the transmission spectrum of BDT at 0V bias in two probe system (the electrodes are gold), the value of it at E_f is larger than experiment about two order of magnitudes due to the underestimation of HOMO-LUMO gap in DFT calculation. So I wonder if DFT+U can imporve the calculation, if yes, then I have two questions:
        (1)How to set Hubbard U parameter in ATK? For example, there are 3s3p4d for S atom , shoud we set U parameter only for 3p orbit or for all of these three orbits? And how to choose the value of U?
        (2)When I set 2eV for 3p orbit of S atom, the calculation is hard to converge. Is there any way to make it converge?
   Waiting for your kind reply!
              Thanks!

6

General Questions and Answers / Re: Optimization of two probe system

« on: October 6, 2014, 05:14 »

You should choose a good initial structure to get a good final optimized structure.

for example
1.  Optimize the molecule part.
2.  Make bulk like structure like [Au-bulk]-Molecule-[Au-bulk] with optimized molecule
3.  Change the Au-Molecule distance, calculate the total energy.  And  you can find the best distance between Au surfaces and S atoms.
4.  Using the lowest energy structure of 3, make 2prove structure.
5   Starting with 4 structure start the optimization of 2 probe.

 

  Thank you for your kind help. One more question, can you tell me the reference energy level to Au electrodes in the modeling system? It really confuses me. Thanks again.

7

General Questions and Answers / Optimization of two probe system

« on: October 5, 2014, 07:17 »

Hi,
   I am a new user of ATK and encounter problems when optimize a two probe system. The py file is attached. After optimization,the molecular structure  in center region gets twisted (the picture is attached) and the distance between S atom and Au surface is 2.26A. That is too long for S atom to adsorb in the hollow site. I just wonder whether the parameters I choose or the initial structure I set are proper. By the way, when I check the log file, I find the Fermi level of Au electrode is -0.025901eV. I want to ask what the reference energy level to this value is. The ATK version is 13.8.0.
    Thanks for your kind help. I look forward to your reply.