Electron Difference Density MGGA for NiGe

[Dhirendra]

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.

[Daniele Stradi]

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

[Dhirendra]

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

[Dhirendra]

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

[Jess Wellendorff]

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.

[Dhirendra]

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

[Jess Wellendorff]

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

[Dhirendra]

Jess,

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

[Dhirendra]

Jess,

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

[Jess Wellendorff]

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

[Dhirendra]

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

[Jess Wellendorff]

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

[Jess Wellendorff]

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?

[Dhirendra]

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.