Messages

151

General Questions and Answers / Re: Calculation using Primitive cell or Conventional Cell

« on: February 6, 2017, 14:09 »

The electronic band structure of the conventional unit cell differs from that of the primitive because their lattice vectors, and the associated Brillouin zones determined from these lattice vectors, are different. As a consequence, the folding of the electronic bands in the Brilluoin zone is different. This is why you observe two different band structures.

The Brillouin zone reduces to only 2 points in the conventional unit cell because when you transform the cell from conventional to primitive, the lattice type is changed by default to "unit cell". You can select an appropriate lattice type by clicking on the Lattice Parameters... option in the Builder.

Regarding the band gap, are you sure you are using exactly the same parameters for the two calculations?

The unit cell to be used really depends on the type of properties you are interested in.

152

General Questions and Answers / Re: advice on basis set/pseudo for complex molecule

« on: February 6, 2017, 12:51 »

Hi Cyrille,

Regarding the basis set you are using, sometimes SZP may be a little bit too small as a basis set for carbon-based systems (for example, the lattice constant of graphene is too large using SZP). The Jahn-Teller effect might be due to this.

However, these things are very basis-set dependent. I suggest that you try with your basis set, and if you still have problem you try to use:

(i) DZP on all atoms
(ii) DZP on C,N,H,B and SG15-M on Fe

Regarding the Mesh cutoff, 150*Hartree is probably over-converged. Normally we suggest 75*Hartree for the FHI basis and 100*Hartree for the SG15 basis. I have tested a few phonon calculations and the forces are normally converged with these cutoff. I suggest that you either use these cutoff (or maybe 100*Ha for FHI and 125*Ha for SG15 if you really want to stay on the safe side) and then eventually increase it further to double check the total energies once you have converged the geometries.

Regards,
Daniele.

153

General Questions and Answers / Re: MoS2 Bandstructure

« on: February 6, 2017, 11:42 »

I checked, it is related to the actual combination of basis set and pseudopotential that you are using.

You can get a direct band gap monolayer MoS2 either using the LDA xc-functional and the DZP basis set, or using the PBE xc-functional and the OMX-Medium basis set.

154

General Questions and Answers / Re: Viewing the final coordinates of QE relax

« on: February 3, 2017, 15:40 »

Hello,

No unfortunately that cannot be done at the moment, but of course we are aware that the functionality is important and will be included as soon as possible in the QE plugin.

Regards,
Daniele

155

General Questions and Answers / Re: advice on basis set/pseudo for complex molecule

« on: February 3, 2017, 15:28 »

Hi Cyrille,

Yes, you can mix different potentials/basis sets. So it is definetly possible to use SG15-M on Fe and SZP on C,H,B,H.

Maybe you can also try DZP on C,H,B,N,Fe. Compared to SG15-M, the DZP basis is less extended, and therefore will computationally more efficient than a full SG15-M basis on all atoms.

What is the difference w.r.t. QE? Do you have some reference we can compare against?

Regards,
Daniele.

156

General Questions and Answers / Re: MoS2 Bandstructure

« on: February 2, 2017, 12:44 »

You did not set the constrain for the cell relaxation correctly. The correct constrain is:
constraints = [FixStrain(x=False, y=False, z=True)]

I this way the in-plane lattice constant of MoS2 is optimized and the interlayer spacing remains fixed, and you get a direct gap. I attach the plot of the bands and the revised script.   

157

General Questions and Answers / Re: MoS2 Bandstructure

« on: February 2, 2017, 11:37 »

Strange, the script looks OK, I will test it.
Daniele.

158

General Questions and Answers / Re: perovskite CH3NH3PbI3 study througk ATK-VNL

« on: February 2, 2017, 11:37 »

Hello,

of course it can be done. ATK-VNL comes with a practical crystal builder in which one can construct complex crystalline systems by entering the crystallographic coordinates:
http://docs.quantumwise.com/tutorials/crystal_builder/crystal_builder.html

Once the crystalline system is specified, it is also possible to study its band structure:
http://docs.quantumwise.com/tutorials/crystal_bandstructure/crystal_bandstructure.html

Regards,
Daniele.

159

General Questions and Answers / Re: How to build nickel-CNT composite

« on: January 27, 2017, 09:42 »

Could you add more information?

If you created the BulkConfiguration for the Ni surface and for the nanotube as described in detail in the first sections of these two tutorials,

http://docs.quantumwise.com/tutorials/work_function_ag_100/work_function_ag_100.html
http://docs.quantumwise.com/tutorials/vnl_cnt_device/vnl_cnt_device.html

then you should be able to combine them together using the Interface Builder:

http://docs.quantumwise.com/technicalnotes/interface_builder/interface_builder.html

160

General Questions and Answers / Re: How to manipulate MoS2 single layer into a rectangular device structure?

« on: January 27, 2017, 09:27 »

1. To obtain a rectangular cell, open Bulk Tools -> Supercell the Builder, change the crystalline system to "Base-Centered Orthorombic" and click transform.
2. You can use the "Swap axes" tool in Bulk Tools -> Swap axes
3. I guess this is because the rotated cell is not commensurate with the unit cell

To make an MoS2 device, apply 1. and 2., then repeat the cell along the C direction (Bulk Tools -> Repeat), then use "Device from Bulk".

161

General Questions and Answers / Re: How to calculate Refractive index, Absorption coefficient from Dielectric values

« on: January 24, 2017, 09:54 »

I think that at the bottom of the tutorial there is a full section with a script, I cite literally:

"The script below calculates the absorption coefficient and refractive index from the dielectric constant and plots it as a function of the wavelength."

162

General Questions and Answers / Re: How to calculate Refractive index, Absorption coefficient from Dielectric values

« on: January 24, 2017, 09:40 »

http://docs.quantumwise.com/tutorials/optical/optical.html

163

General Questions and Answers / Re: Germanium nanowire

« on: January 23, 2017, 09:10 »

Great! Thanks for reporting 

164

General Questions and Answers / Re: problem displaying Bandstructure of Graphene: VNL as QE GUI

« on: January 18, 2017, 10:25 »

Yes there is a bug due to a difference between how QE and VNL interprets numerically the values of the k-points: VNL identifies automatically the K-point with a precision of 10-8, but QE prints out the k-points in the band structure only with 4 decimals. Hence, the K-point is not identified as an high symmetry point and it is not printed out.

I have already filed a bug which will be corrected asap.
I will post here when the bug is corrected.

165

General Questions and Answers / Re: VNL and phonon band QE

« on: January 17, 2017, 14:37 »

Hello, no this functionality is not implemented yet. However, it is still possible to write your own AddOn to import the phonon band structure as a band structure object in VNL

http://docs.quantumwise.com/tutorials/addons/addons.html