Messages

91

General Questions and Answers / Re: Fermi velocity in zigzag graphene nanoribbon

« on: May 6, 2020, 17:18 »

The answer really depends on what you want. In this ultra narrow ribbon the Fermi level crosses several bands, unlike pristine graphene with its 6 conical (equivalent) points.

92

General Questions and Answers / Re: scattering matrix

« on: May 6, 2020, 10:13 »

I guess it is not currently possible to compute this phase shift as it relies on the calculation of scattering states (eigenchannels) that are not explicitly computed in the NEGF approach, unlike the wave function method adopted in the cited paper.

93

General Questions and Answers / Re: mobility of graphene

« on: May 6, 2020, 09:13 »

Seeing your script could be helpful. Also, do you mean the status of the job is finished in the Job Manager, whereas the job is still running on a remote cluster? Do you actually use a remote cluster for this calculation? Then you may also have additional output files, e.g., error file or something like that.

94

General Questions and Answers / Re: phonon spectral function

« on: May 5, 2020, 11:36 »

Please clarify how you define the weight of phonon bands and what is your system(s) of interest.

95

General Questions and Answers / Re: refractive index of graphene

« on: April 25, 2020, 02:59 »

I used GGA and HSE06 to calculate refractive index of graphene. The results show approximately n=1.7 for visible light range.

Given that periodic boundary conditions are imposed in all 3 space directions, did you check how GGA-calculated refractive index value depends on the vacuum thickness in-between graphene monolayers in neighboring cells in out-of-plane C-direction? Now it is set to 12 Ang in the script. What would happen if it is 24 Ang, for instance?

96

General Questions and Answers / Re: Molecular Dynamics at low temperature

« on: April 23, 2020, 11:08 »

I confirm special thermal displacement method takes zero-point motion into account. That can be seen explicitly if one generates an STD structure at 0 K, which will have all the atoms displaced from their equilibrium positions, even so the temperature is at absolute zero.

97

General Questions and Answers / Re: Transmission Spectrum for superconductors

« on: April 17, 2020, 16:11 »

I do not think DFT-NEGF mean field  methodology allows describing superconductors.

98

General Questions and Answers / Re: Molecular Dynamics at low temperature

« on: April 17, 2020, 16:06 »

This method does not account for zero-point motion that might be important for low temperatures.

99

General Questions and Answers / Re: Query regarding HSE functional

« on: April 14, 2020, 09:04 »

Your calculation looks healthy. I see no issue: stress and forces are gradually decreasing.

I do not think you need 2 k-points in the out-of-plane direction, i.e., nc can probably be set to 1. I would also reduce the number of k-points in the other directions, as your in-plane unit cell looks relatively large. I would first test basic convergence with respect to k-point sampling using GGA functional that is much cheaper, and do it for different k-point grids to see how much that affects the results. You may also do HSE optimization with less k-points and then increase the grid density when computing the band structure/band gap.

Is there any band gap in this structure when using GGA? You may also check whether your GGA-optimized structure and current HSE-optimized structure have similar GGA (or HSE) gaps or band structure in general. That would allow you checking if using HSE is actually important for relaxation or GGA works good enough.

100

General Questions and Answers / Re: Application of an external magnetic field

« on: April 10, 2020, 13:36 »

It is not possible.

101

General Questions and Answers / Re: Bilayer Graphene (AB stacking)

« on: April 7, 2020, 23:41 »

This tiny shift looks very much within numerical noise. You cannot just set the Fermi level, as the Fermi level position is actually computed, and this is where this noise comes from. You may try increasing the k-point sampling to increase the accuracy.

102

General Questions and Answers / Re: Query regarding HSE functional

« on: April 7, 2020, 09:48 »

HSE functional is computationally much more demanding than GGA. So, that is expected that the computational time has significantly increased. It might be that you could optimize your settings to make it somewhat faster. Could you post your python script and related log-file to see if everything is fine with the current calculation, in principle?

Note again that the computational cost for HSE calculations is very much dependent on the number of k-points used for sampling the Brillouin zone, as well as on the wave function cutoff and the number of unoccupied bands included in the calculation. 

103

General Questions and Answers / Re: Virus detection

« on: April 2, 2020, 14:56 »

In general, if you have a cif-file in your project folder, it should then appear in the QuantumATK NanoLab GUI. After selecting this cif-file in the GUI, a configuration object appear on the LabFloor. This object can then be drag-and-dropped on the text Editor available in the GUI and the corresponding file can be saved as a python file. See also this https://docs.quantumatk.com/tutorials/quantumatk_io_import/quantumatk_io_import.html for other options.

104

General Questions and Answers / Re: bandstrucutre of graphene

« on: April 1, 2020, 21:59 »

You change not only the unit cell of graphene in the direct space, but also in the reciprocal space, i.e.,  Brillouin zone (BZ) is no longer hexagonal, so you should search in the new BZ for what was the K-point in the original hexagonal cell.

Also, if this nanoribbon has a finite width with edges, the system may have a band gap.

105

General Questions and Answers / Re: Virus detection

« on: April 1, 2020, 21:52 »

The cif-file is 606Mb. Do you really want to model a structure with millions of atoms?