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QuantumATK => General Questions and Answers => Topic started by: Dhirendra on January 4, 2016, 01:35

Title: Electron Difference Density MGGA for NiGe
Post by: Dhirendra on January 4, 2016, 01:35

Hi,

I am doing the MGGA study on NiGe (from database).  A NiGe slab is simulated with GGA(blue) and MGGA(green) and then macroscopic average of the electron difference density is plotted.  A significant EDD can be seen at the edges of the slab, but in GGA and MGGA, signs are opposite.  The MGGA c parameter is set to be 0.97. Is MGGA reliable then? I am attaching the .py file of the simulations.

Title: Re: Electron Difference Density MGGA for NiGe
Post by: Daniele Stradi on January 4, 2016, 10:56

Dear Dhirendra,

I would not use the MGGA functional for a slab calculation because it might lead to unreliable results. The reason is that the MGGA exchange potential diverges in the limit of a vanishing electronic density (see Equation 1 in PRL 102, 226401).
This might give problems for slab calculations as you have two surfaces where the density gradually tends to zero as you move away from the surface.

 Daniele

Title: Re: Electron Difference Density MGGA for NiGe
Post by: Dhirendra on January 4, 2016, 12:48

Thanks Strandi, but I am fixing the c parameter which is recommended from Quantumwise for the slab simulations.

Title: Re: Electron Difference Density MGGA for NiGe
Post by: Dhirendra on January 5, 2016, 21:33

How is n^atom(r-R_u) is calculated in the "Background Information" of the manual? It will help me understand about the above behaviour.
http://www.quantumwise.com/documents/manuals/latest/ReferenceManual/index.html/chap.atkdft.html#sect1.atkdft.background

Title: Re: Electron Difference Density MGGA for NiGe
Post by: Jess Wellendorff on January 6, 2016, 10:10

That term is simple a superposition of the atomic pseudopotential densities. Any charge density reorganization due to many-body interactions is then the difference between the DFT self-consistent charge density and that superposition of densities. However, please note the comments below by Daniele: The MGGA exchange potential may diverge in the vacuum region of a slab calculation. This is the main difference to LDA/GGA slab calculations.

Title: Re: Electron Difference Density MGGA for NiGe
Post by: Dhirendra on January 6, 2016, 20:45

Jess,
Here is the NiGe/Ge contact in the device configuration. MGGA(blue) and GGA(green) has a sign change.

Title: Re: Electron Difference Density MGGA for NiGe
Post by: Jess Wellendorff on January 7, 2016, 08:28

Yes, I see the sign change. I will need to see your script before I can say anything more concrete about this.

Title: Re: Electron Difference Density MGGA for NiGe
Post by: Dhirendra on January 7, 2016, 11:15

Jess,

I am sending you the .py file in personal message.

Title: Re: Electron Difference Density MGGA for NiGe
Post by: Dhirendra on January 7, 2016, 11:24

Jess,

How do I attach documents in PM user interface? Is there any other mode to send you the file?

Title: Re: Electron Difference Density MGGA for NiGe
Post by: Jess Wellendorff on January 8, 2016, 08:24

You can use the "Attachments and other options" functionality just beneath the area used for writing posts...

Title: Re: Electron Difference Density MGGA for NiGe
Post by: Dhirendra on January 8, 2016, 08:47

Jess, is it possible to share the file with you privately?

Title: Re: Electron Difference Density MGGA for NiGe
Post by: Jess Wellendorff on January 13, 2016, 14:19

Yes, at "jess.wellendorff@quantumwise.com"

Title: Re: Electron Difference Density MGGA for NiGe
Post by: Jess Wellendorff on January 14, 2016, 12:00

I have tested the device calculation with both GGA and MGGA. For purposes of testing I have significantly reduced the system size. The attached plot shows the ElectronDifferenceDensity for MGGA and GGA. I do not see any change of sign. How did you plot the figure you show?

Title: Re: Electron Difference Density MGGA for NiGe
Post by: Dhirendra on January 14, 2016, 19:23

I am calculating the microscopic average of electron difference density. If you plot that then you would be able to see the sign change. I have attached the script. You have to edit it little bit. The averaging window is taken as lattice constant of Ge.