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Messages - Petr Khomyakov

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1
Questions and Answers / Re: I-V of graphene nanosheet
« on: June 3, 2021, 01:14 »
You would need to design proper electrodes when applying bias voltage, e.g., by doping source and drain, see an example on how to introduce doping in https://docs.quantumatk.com/tutorials/silicon_nanowire/silicon_nanowire.html (Section: Doping the Si(100) wire).

Also, you may consider using IVCharacteristics study object to do device calculations, see https://docs.quantumatk.com/tutorials/ivcharacteristics/ivcharacteristics.html.

2
Questions and Answers / Re: I-V of graphene nanosheet
« on: June 1, 2021, 12:04 »
Could you post a script used for this calculation? For example, if your graphene ribbon is intrinsic and has no doped-graphene based or actual metal source/drain electrodes attached, then the IV characteristics will depend on the device central region length.

3
For band structure analysis to identify those bands that arise from particular parts of the system, you may use projections on elements and tags  in FatBandStructure, https://docs.quantumatk.com/manual/Types/FatBandstructure/FatBandstructure.html and https://docs.quantumatk.com/manual/Types/Projection/Projection.html#projection-c. The fat band structure is available through the GUI as an analysis object, but can also be edited through scripting indeed.

4
Questions and Answers / Re: Homo-Lumo energy
« on: May 27, 2021, 12:40 »
For a given electron temperature and fixed total number of electrons N_electrons, the Fermi level/energy E_F are actually given by a different equation, N_electrons = Integral DOS(E) f_FD(E-E_F) dE, where f_FD is Fermi-Dirac distribution, https://en.wikipedia.org/wiki/Fermi_level. It does not need to be  always in the middle of the gap.

5
Questions and Answers / Re: Homo-Lumo energy
« on: May 20, 2021, 12:40 »
This is more chemistry terminology assuming a  discrete energy spectrum given by atomic or molecular orbitals, whereas in solid state we are talking about a continuous energy spectrum with an energy gap and possible discrete levels (e.g., when there are localized states due to defects or dopants). I guess bottom of CB and top of VB could sort of be seeing as LUMO and HOMO, respectively, provided that the Fermi level is in the gap, which would be the case for intrinsic semiconductors and insulators.

6
Questions and Answers / Re: SQS Alloy
« on: May 17, 2021, 13:57 »
The theory of SQS and guidelines for generating these structures are given in this papers A. Zunger, S.-H. Wei, L. Ferreira, J.E. Bernard, Special quasirandom structures, Phys. Rev. Lett. 65 (3) (1990) 353; S.-H. Wei, L. G. Ferreira, James E. Bernard, and Alex Zunger, Phys. Rev. B 42, 9622 (1990), see also https://docs.quantumatk.com/manual/Types/EvolutionarySQS/EvolutionarySQS.html.

As a general guideline, increasing accuracy of SQS requires larger supercells, higher Max Expansion Order (the current default is likely good enough so), using larger number of generations, using the largest possible cluster diameter, see a png-file enclosed. The goal is to get error for correlation functions as small as possible for a given expansion order and max cluster diameters (default is likely good enough).  Note that setting very strict parameters might lead to significant increase of computational time. Again, quality of SQS is quantified in terms of correlation function errors.

7
Questions and Answers / Re: Doping in Device Simulations
« on: May 3, 2021, 12:59 »
It would instructive to see Hartree Difference Potential / PLDOS  for the device calculated using the tutorial vs. new doping level adopted in your calculation.

Note that for low-dimensional systems such as films and nanowires one should use charge per atom units when setting doping level, meaning that one should recalculated e/cm**3 for bulk system to e/atom for low dimensional. The doping assumed in your calculation seems to be significantly larger than that adopted in the tutorial.

8
If you refer to inset (a) in the figure, creating tags for top, middle and bottom layers and then doing projection on tags is the right thing to do.

Please could you explain what exactly failed in projecting on the tags - you may also post the corresponding picture of the band structure, script and log of the calculation for the sake of completeness?

9
Questions and Answers / Re: lattice parameters
« on: May 3, 2021, 12:38 »
There are typically 2 choice (not limited by no means so): measured value or equilibrium value calculated with a particular Calculator using OptimizeGeometry, https://docs.quantumatk.com/manual/Types/OptimizeGeometry/OptimizeGeometry.html, https://docs.quantumatk.com/tutorials/uniaxial_biaxial_stress/uniaxial_biaxial_stress.html. In the latter, the target stress should be set to zero to get an equilibrium lattice parameter.

In some cases, one may want to strain crystal structure to match it to another material, see https://docs.quantumatk.com/tutorials/ag_au_interface/ag_au_interface.html.

Note that using measured value would like to mean introducing some strain into the corresponding structure, unless the calculated one is exactly as  the measured one.

10
Questions and Answers / Re: Absorption coefficient
« on: March 24, 2021, 14:40 »
An additional comment is that the optical spectrum analysis object assumes an array of molecules, e.g., molecular crystal, rather than a single molecule. One can still do the calculation for a single molecule, but then there will be a dependence of the optical properties on the amount of vacuum around the molecular structure.

11
I do not see anything peculiar in your script. You may try calculating ElectrostaticDifferencePotential for epsilon=1 and epsilon>>1, and compare that for the 2 cases to see if there is any effect of changing epsilon on the electrostatic potential in your calculations. 

12
Please take a look at an example of H2 dissociation energy calculation in the manual https://docs.quantumatk.com/manual/Types/TotalEnergy/TotalEnergy.html. Enclosed you may also find a script that does separation distance optimization in an automated manner, and H2 do not dissociate upon optimization (d_eq~0.76 Ang for the computational settings chosen).

So, these are 2 examples of successful optimization of H2.

13
If you do not see Counterpoise in the Calculator, see a png-image enclosed, it means you have to do it manually as described in the tutorial.

14
For DFT calculations, I would start with GGA-PBE. You may also need this correction, https://docs.quantumatk.com/tutorials/dispersion_corrections_and_bsse/dispersion_corrections_and_bsse.html, this can now be set in the GUI. Also, you might consider including van der Waals interaction. But that needs to be checked.

15
General Questions / Re: single electron transistor
« on: January 12, 2021, 15:31 »
I have done that tutorial and the Quantum wise tutorial "Benzene Single-Electron Transistor".
Which tutorial are you referring to? Could you post a link?

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