Messages

16

General Questions and Answers / Re: Regarding bandstructure difference between unit cell and repeated cell

« on: September 1, 2020, 13:36 »

The brillouin zone points in the repeated cell are not high-symmetry points in the same way as for the minimal cell, so the band paths are different. No surprise there. I suggest you look are the DOS instead; if you normalize properly (e.g. on a per-atom basis), and use well-converged k-point grids for DOS, the DOS should be similar no matter the repetition. 

17

General Questions and Answers / Re: equilibrate water molecules in a box

« on: May 13, 2020, 14:22 »

In addition to using Packmol to construct the box with randomly positioned H2O molecules, a simple Force-Field based geometry optimization (minimization of both forces on atoms and stress on the supercell) might give a good starting point for DFT.

Flow:
1. use Packmol to construct the box with randomly positioned H2O molecules (perhaps aim for experimental density of molecules).
2. change cell type from UnitCell to SimpleCubic.
3. use ForceField calculator and OptimizeGeometry to relax forces and stress (take care to select a force field that actually works for water).

Example script attached. Runs in 2 mins on a laptop. Resulting water structure looks pretty OK to me, especially with the expected directional non-covalent bonding between H and O on different molecules.

If you believe finite-temperate molecular dynamics is needed to get better or more realistic water structure, that is also an option, but less simple than flow given above.

18

General Questions and Answers / Re: Analysis from File in NanoLab-GUI

« on: April 17, 2020, 14:20 »

Dear Jie Jiang,
we are looking into this. Get back to you soon.
Regards,
Jess

19

General Questions and Answers / Re: SOGGA.PBE

« on: January 2, 2020, 09:57 »

1) spin-orbit coupling breaks time-reversal symmetry, so k-point reduction from time-reversal must be switched off.
2) looks like with LCAO uou can also use OMX for SOGGA. Not sure if OMX is available for PlaneWave also.

We have no plane-wave tutorial specifically for Q-2019.12 release. But we do have this one: https://docs.quantumatk.com/tutorials/pw_intro/pw_intro.html

20

General Questions and Answers / Re: Measuring lattice constant

« on: January 2, 2020, 09:52 »

It will be printed in the log file. But most easy to select the optimizwed BulkConfiguration on the Lab Floor, and use Text Representation widget to see info on lattice constants, atomic positions, etc.

21

General Questions and Answers / Re: The construction of a two-dimensional material

« on: August 12, 2019, 15:23 »

Dear Bertha,
the fuzz factor simply determines how close two atoms must be in order for the GUI to draw a bond between them, so it is safe to tune it a bit according to chemical intuition. As for the k-point sampling along direction A, the equilibrium lattice constant should converge to a constant value as k_A increases, so you should probably check that.

22

General Questions and Answers / Re: DFT+MD with electric field

« on: May 20, 2019, 15:15 »

The DFT-LCAO engine does not support partil charge analysis. This is also clear if you search for PertialCharges in the online documentation:
https://docs.quantumatk.com/manual/Types/PartialCharges/PartialCharges.html
"... an attached classical calculator"

So you need to define a ForceField calculator and attach it to the bulk configuration.

23

General Questions and Answers / Re: How to calculate current value from the finite-bias transmission spectrum

« on: May 20, 2019, 14:17 »

Load the TransmissionSpectrum and use the "current" method to get the current at that bias:
https://docs.quantumatk.com/manual/Types/TransmissionSpectrum/TransmissionSpectrum.html#NL.Analysis.TransmissionSpectrum.TransmissionSpectrum.current

24

General Questions and Answers / Re: Band structure Energy

« on: May 20, 2019, 14:16 »

It is very simple: The band structure plot is Energy vs. k-point position in the BZ, so the x-axis is a route in the BZ. The y-axis is Energy referenced to the Fermi level, so -2 eV means the energy is 2 eV below the Fermi level. Just a standard band structure plot.

25

General Questions and Answers / Re: how to use ParallelDevicePerformanceProfile

« on: March 25, 2019, 11:08 »

See also the documentation for that functionality: https://docs.quantumatk.com/manual/Types/ParallelDevicePerformanceProfile/ParallelDevicePerformanceProfile.html

26

General Questions and Answers / Re: Bulk MoS2 band gap

« on: January 21, 2019, 10:27 »

0.89 eV is what you would expect for bulk MoS2 if using a LDA or GGA density functional, there are many examples in literature, see e.g. https://file.scirp.org/pdf/Graphene_2014102014384642.pdf

27

General Questions and Answers / Re: Calculation using STD method cannot get converged

« on: September 24, 2018, 21:39 »

Yes, non-STD means in this case non-interacting.
No, improved screening towards the electrodes are accomplished by (1) increasing the central region size and/or (2) doping the device.
The STD method relies in the dynamical matrix, so the simulation engine must provide reliable forces. Tight-binding models are in general not suited for this. On the other hand, classical force fields, as implement in the ATK-ForceField calculator, are idel for the purpose (i.e. for computing the dynamical matrix), provided that a good force field exists for the system of interest. I believe this is the case for a graphene-based device.

28

General Questions and Answers / Re: Issue related to Graphene script in espresso.in format

« on: September 13, 2018, 09:49 »

The problem has nothing to do with the QuantumATK software. It is most likely because you do not set the QE relaxation parameter "cell_dofree", which by default is 'all' in quantum espresso. See http://www.quantum-espresso.org/Doc/INPUT_PW.html. If you have questions regarding such QE settings, I suggest you post them on the QE users mailing list: users@lists.quantum-espresso.org

Your VNL-ATK 2015 did what you asked it to do (produced a simple QE input file), and it is now your own responsibility to adapt it correctly to your specific needs.

Kind regards,
Jess Wellendorff

29

General Questions and Answers / Re: Band structure of electrode at finite bias voltage

« on: June 20, 2018, 14:16 »

The band structure of a device electrode does not change under finite-bias NEGF calculations, only the Fermi level is shifted because of the applied bias. In fact, the electrode electronic structure is calculated before the NEGF calculations starts, and is fixed for the entire NEGF calculations, also under finite bias (except for a possible shift of the Fermi level).

I suggest you use the ProjectedLocalDensityOfStates analysis to investigate the effect of biasing your device. See e.g. this tutorial:
https://docs.quantumwise.com/tutorials/inas_p-i-n_junction/inas_p-i-n_junction.html#band-gap-and-band-edges. Perhaps consider also consider computing the Voltage Drop: https://docs.quantumwise.com/tutorials/atk_transport_calculations/atk_transport_calculations.html#voltage-drop

30

General Questions and Answers / Re: Flat band and effective mass

« on: March 22, 2018, 13:03 »

Are you calculating the effective mass along the correct/expected direction? (in the M-K direction in reciprocal space)