Messages

16

General Questions and Answers / Re: Bulk MoS2 band gap

« on: January 18, 2019, 14:05 »

1.2 eV is the experimental value for bulk MoS2 (Zhiming, MoS2 Materials, Physics and devices).  I have converged the calculation.

17

General Questions and Answers / Bulk MoS2 band gap

« on: January 18, 2019, 07:19 »

Hi,

I am using QuantumATK 2017.2 version. I am doing band structure calculation for bulk MoS2. I am using ATK-DFT: LCAO, GGA-PBE exchange-correlation, 75 Ha mesh cutoff, 8x8x8 k point, OMX pseudopotential with Medium basis set. But I get band gap 0.89 eV rather than 1.2 eV.

18

General Questions and Answers / Geometry optimization

« on: November 6, 2018, 16:16 »

When evaluating the band structure or doing any calculation in ATK, why is it necessary to do a geometry optimization? The unit cell dimensions and distance between atoms is already determined from X-ray diffraction experiments and we have a list of all those data. Then why is geometry optimization required?

Can someone explain the process what background calculation is done at the back end to optimize the geometry. I want to know after each geometry is the KS equation solved and Hamiltonian determined to find the lowest energy/minimum force between atoms?

19

General Questions and Answers / Re: Decreasing number of bands in bandstructure

« on: September 21, 2018, 19:56 »

I changed the unit cell to hexagonal and the problem was resolved. Thanks !!

20

General Questions and Answers / Re: Decreasing number of bands in bandstructure

« on: September 18, 2018, 08:11 »

hi,

I tried with 1 x 1 unit cell of monolayer MoS2 with 5 x 5 x 1 k point sampling. Still for band structure calculation its showing me only G, Z. I am unable to choose G-M-K-G. I found out that when I delete one of the layers of unit cell from database and do band str. calculation, I am able to see G-M-K-G. But when I stretch the uni cell along c axis to create a vacuum layer, then I see only G, Z option. How to get rid of this?

21

General Questions and Answers / Re: Decreasing number of bands in bandstructure

« on: September 10, 2018, 10:11 »

Yes I took the unit cell parameters from the database

22

General Questions and Answers / Re: Decreasing number of bands in bandstructure

« on: September 10, 2018, 09:16 »

First I took 1 unit cell of MoS2. Deleted the second layer and then selected Repeat option. A: 4 and B: 4. C axis is perpendicular to monolayer MoS2 -plane

23

General Questions and Answers / Re: Decreasing number of bands in bandstructure

« on: September 10, 2018, 06:19 »

For a 4 x 4 supercell of mono-layer MoS2, I am unable to choose the G-M-K-G brillouin zone route. Is it because the system is now hexagonal and do I need to convert it to a rhombohedral system? The c axis is now perpendicular to the plane of MoS2.

24

General Questions and Answers / Re: Decreasing number of bands in bandstructure

« on: September 8, 2018, 19:38 »

Thanks for the valuable info Petr. Just wanted ask if my supercell is 4 x 4 size, then how to we choose the appropriate k space sampling grid? I generally see in papers they use 3 x 3 x 1 or 9 x 9 x 1. So what is the apt choice of  k space sampling for a 4 x 4 MoS2 supercell.

25

General Questions and Answers / Re: Decreasing number of bands in bandstructure

« on: September 8, 2018, 06:28 »

I am using QuantumATK 2018.06. I did my calculation with a 3 x 3 supercell. I was using plane wave DFT option with LDA and HGH pseudopotential (Tier 0 for both Mo and S). I set broadening as 300 K and wavefunction cut-off as 50 Hartree. I took a 3 x3 x 1 k space with bands per electron = 1.2.  I also increased the unit cell spacing to create a vacuum on both sides of a MoS2 monolayer. I have also optimized the geometry and then did the band structure calculation. Did you optimize the geometry or just used the experimental Wycoff positions for band structure calculation?

26

General Questions and Answers / Re: Decreasing number of bands in bandstructure

« on: September 7, 2018, 14:46 »

Thanks a ton Ulrik !!
Although I still didn't get the direct band gap at K-K point of BZ, I can see the valley like structures at least. Will try other combinations of XC and basis sets.
Thanks again.

27

General Questions and Answers / Re: Decreasing number of bands in bandstructure

« on: September 5, 2018, 17:59 »

I have tried all sorts of zoom but still can't see the valleys in the band diagram. Could you please run the input file and see?

28

General Questions and Answers / Re: Decreasing number of bands in bandstructure

« on: September 5, 2018, 14:08 »

This is what I get even after zooming. I have been trying with various combination of XC functional and basis sets. This is with LDA HGH. But I don't see the valley like structure. I don't understand why this is happening. This is for monolayer MoS2 (Supposed to give direct band gap at K point of 1.8 eV)

29

General Questions and Answers / MoS2 orientation for band structure

« on: September 3, 2018, 15:50 »

If i am doing only band structure calculation, is it necessary to align the z direction in MoS2 plane?
I am referring to step 3 in Geometry for MoS2, https://docs.quantumwise.com/tutorials/transmission_gr_mos2/transmission_gr_mos2.html

30

General Questions and Answers / Re: Decreasing number of bands in bandstructure

« on: September 3, 2018, 14:50 »

I selected the HGH pseudopotential Tier 2 and Tier 3 for S and Mo respectively. This time I did not optimize the geometry and used the unit cell of 2H MoS2 by deleting the second layer. I increased the unit cell by 15 A on both sides (this adds vacuum ??). But still bands are flat and I don't see valley like structures. When I zoom in at K point, the bands are more or less flat.

Please help !!